Let's investigate the msytery of the U.A traitor. I'm going to walk you through the evidence and my analysis of what's been presented and introduce you to the suspects, then I'll let you draw you're own conclusions.
I look forward to getting you're feedback.
What do we know about the traitior?
There's strong circumstantial evidence that there is indeed a traitor at U.A, either among the staff or the students. We know the teacher's schedules are a closely kept secret, but during the USJ attack Kurogiri mentioned that they had received a copy of it.
This could have been stolen by a student when the teachers were distracted by the media invading the campus, or it could have just been given to them by a teacher. I think we can confidantly rule out All Might, Eraserhead, and Thirteen as suspects.
Episode 39 aka the most important filler episode ever
"Season 3 Episode 1: Game Start" was an anime original episode dedicated to recapping the plot of the past two seasons.
Near the end of the episode we see Ereasurehead telling Vlad King that the training camp location was changed at the last minute as a precaution against info being leaked to villains. At the end of the episode we get a scene with Tomura Shigaraki and he praises his his master saying how "easiy he found the destination of those guys. I couldn't find no matter how hard I looked."
Suspect List for Students
There's two major questions you need to answer if you're going to accuse one of the characters of being the U.A traitor, those questions are "Why?" and "How?".
If we take episode 39 as canon to the manga then the first three students on this list are the only ones who could possibly be the traitor, but even if isn't I stil consider the top 3 to be prime suspects.
Suspect 1# Toru Hagakure
If you have any interest in the U.A traitor you've probably heard most of the evidence towards Toru Hagakure before. She has a quirk uniquely suitable for stealth, meaning she could potentially be in any scene and we wouldn't know it.
Toru Hagakure was also absent from class the day that Tomura crumbled the gate and let the Reporters flood the building. In chapter 12 they held elections for a class rep, most students only got one vote because they voted for themselves but Izuku and Momo got more than one.
Izuku got 3 votes (one from himself, the other two were Iida and Ochako), Momo got 2 votes (herself and Shoto). The number of votes and the number of students however do not add up. There's one person whose vote is unaccounted for and that would be Toru.
In the entire series we have never seen her fight a villain. She along with Aoyama were conspiciously absent during the USJ attack and this was even specifically pointed out. I think it's implied that Aoyama was hiding but Toru says she got transported with Shota Todoroki. If she was with him then why wasn't she frozen, and why didn't she say anything to Todoroki?
Remember that time Izuku ran into Tomura Shigaraki at the mall? Do you remember who suggested they go there? Yes it was the Invisible Girl.
If you're familiar with Traitor theories you've probably heard all of this before, but if she is the traitor then I think we also know what her motivation is. During chapter 193 we see that All for One gained people's loyalty and built his empire by taking quirks away from those who saw their powers as a curse and giving quirks to people wo were scared and defenseless or who were simply ambitious.
Toru Hagakure has consistently been portrayed as socially eager and with a desire to stand out, so if she is the traitor then I don't think she's doing it for money or because she has an idealogical goal that aligns with the League of Villains. I thinks he wants All for One to take her quirk away from her.
If Toru Hagakure isn't the traitor then she is a masterful red herring. She's at the top of the list because she had the means, the motive, and the oppurtunity.
Suspect 2# Mezo Shoji
There's numerous reasons to suspect Mezo Shoji of being the traitor. His quirk is great for gathering information and allows him to hear and see things happening far away.
During the USJ attack when Kurogiri was about to stop Tenya Iida from escaping to go get help, Mezo Shoji tackled Kurogiri stopping him from stopping Tenya. Why does this constitute evidence for Shoji being the traitor? Remember that the purpose of the USJ attack was to kill All Might not to kill the kids.
Allowing one of the kids to escape to go get help could have been a smart move tactically since the first hero to arrive would of course be All Might.
During the Summer Training Camp Arc, he got one of his arms severed by Moonfish. This injury was meaningless because his Dupli-Arms can duplicate themselves and basically grow back, but it had the effect or sending Dark Shadow into a rampage.
We later see that his dorm room is mostly empty and while he claims not to be materialistic, we could instead see this as evidence of him being secretive.
"Walls have ears. Doors have eyes
Trees have voices. beasts tell lies
Beware the rain, beware the snow
Beware the man you think you know"
Suspect #3 Koji Koda
Koji Koda's quirk Anivoice allows him to communicate with and control animals, which could also be great for information gathering since he could turn any small insignificant animal into a spy for him. In U.A this quirk has another advantage because the principle of the school is not a human being, the principle is an animal with an intelligent hightening quirk.
The traitor being able to take control of the principle could help explain why Nezu hasn't sniffed the traitor out already.
Koji Koda is an introvert but for a spy this actually makes sense, like Toru Hagakure he's somebody that the audience can forget exists but he's even more invisible since he doesn't even try to stand out.
I know this isn't actually evidence but I'd like to draw attention to Koji Koda's design. He looks like a hulking rock monster. Similar to Alphonse Elric, he could easily be made to look like an intimidating brute. Al from FMA was badass sometimes but other times he was adorable, but with Koji Koda the adorable switch is on almost constantly.
If Koji Koda is revealed to be the traitor then this reveal will be preceeded by a fake-out where they make us think Principle Nezu is the traitor only to reveal Nezu was being controlled by Koda.
Those eliminated as a suspect
Ochako Uraraka is innocent
Uraraka has a very clear cut motivation for being the traitor, she needs the money. Attending U.A and becoming a pro hero could help her in the long run, but we don't know how much it's costing her parents to send her to this school. This motivation would tie thematically into Stain's point about for profit heroes.
If Ochako was the traitor I can imagine her being conflicted but rationalizing her actions. So what if giving this information to the League of Villains causes them to attack USJ and try to kill All Might, if All Might is there then he'll win the day and stop the bad guys won't he? The fact that "everything worked out" in the USJ incident could have reinforced this idea and made Ochako believe she was worried over nothing, which is why she did the same thing again.
Of course the price of victory at the USJ was that All Might was weakened and couldn't stay in his hero form as long but Ochako doesn't know that. If I'm right about the way Ochako rationalizes her trechery then it could tie thematically into Tomura Shigaraki's backstory and his belief that heroes are ultimately dangerous as they make people more complacent.
Even though it would be interesting if Uraraka was the traitor I do not think that she is the traitor. Unlike Toru, Shoji, and Koda she doesn't have a quirk that's suited for espionage and during the time the reporters invaded the campus (the time when the teachers were distracted and a student could have stolent he schedule) we know exactly where Ochako was.
Another piece of evidence against her being the traitor can be found in how poorly she hides her crush on Izuku Midoriya. Ochako Uraraka has been portrayed as extremely honest and blunt and somebody who wears her heart on her sleeves, and in her crush on Deku we see her trying to keep a secret and supress her feelings but sucking at it.
Izuku Midoriya and Tenya Iida are innocent
Aside from Izuku being the main character, he and Tenya Iida have the same alibi as Ochako Uraraka for where they were when the reporters invaded the campus and the schedule would have been stolen.
The non-hero students and the upperclassmen are innocent.
If the traitor is a student it needs to be somebody in class 1A or class 1B, because they were the ones who went to the training camp and were the ones who could have leaked it's location (assuming we ignore the anime original scenes in episode 39). This eliminates Mei Hatsume, Hitoshi Shinso, the big three and basically every other student who isn't a member of class 1A or 1B from the list of suspects.
I originally listed Hitoshi Shinso as a potential traitor because theoretically his brainwashing quirk could allow him to get information out of the teachers, but the more I thought about the limits of his powers the less that made sense.
When Hitoshi controls somebody they feel dazed and look spaced out, but remain largely aware of their current state. Essentially they are caged within their bodies. So if he'd brainwashed one of the teachers then they would know what he's done afterwards. Hitoshi claims that any orders which require his target to think or talk will not work, and if that's true he couldn't have forced them to tell him the schedule or camp location anyway. Hitoshi could of course be lying about that limitation but it's consistent with how his quirk has been portrayed so far in the manga.
All Might is innocent
All Might's innocence should go without saying, since he is the arch-enemy of the League of VIllains and the one they are trying to kill.
Shota Aizawa and Thirteen
I think his actions and the injuries he sustained at the USJ attack are enough to clear him of suspicion, he stopped Tomura from killing Froppy when he didn't need to. The injuries he sustained in this attack reduced how long he could use his quirk.
Now we've seen from Izuku that some people are so dedicated they will horrible injure themselves to acheive a goal, but there's another reason to think Shota Aizawa is innocent. If Shota Aizawa wanted to take down All Might he could have done so on his own at any time.
Thirteen also sustained injureis during the USJ attack and fought to save the kids, but we also have another reason to think she is innocent. Thirteen knew All Might wasn't going to be able to make it to the USJ but when the villains arrived they expected All Might to be there.
Next week I'll publish part 2, which will include an analysis of the teachers.