He has expelled 154 students. At most he could expel 20 a year which would be roughly 8 years. but I doubt it always came that close to 20. if its been 10 years thats still about 15 a year but i guess that makes more sense. The only other possibily is that each time someone is expelled someone takes his place so 20 isn't actually the cap. In that case with him expelling 20 all at once last year and the potential of 20 more being expelled he could have expelled something like 30 a year making him only a teacher for 5 years. Once we figure this out then we can figure out the timeline between the two series because in 6+ alpha of the vigilante series it states that it takes place a year prior to Aizawa starting as a teacher. This gives that series plenty of time to flesh out Stendhal/stein and showcase Ingeniums career. depending on the time allotted.