Board Thread:Questions and Answers/@comment-1471207-20170807213711/@comment-33005802-20170901150148

We can use Stain to help us here. We know he dropped put of his freshman year, spend a bit of time preaching about his ideals, going off to train as a killer for a decade, then being a vigilante, where we see him as Stendhal in the spin off. Taking all this into consideration, I'd say at this point he's 27. In the manga, he's 31 (the same age as Midnight) and Aizawa is 30, and by this, one can make an educated presumption that Aizawa and Midnight have been teaching for three years.

With regards to the amount of students expelled, it's highly likely Aizawa taught multiple classes. If he has been teaching for three years, that gives him a total of new five yr groups(*) that he has seen (it's unlikely he'll expel anyone he's taught for over a year, so this figure comes from the first three year groups when he joins, the freshman yr of his second year and the freshman yr of his third - Deku's yr). There are also multiple classes in each yr group, A-K. Assuming each class has 20 students, that's 220 students per yr group, and 1100 students in total that he could've taught. He obviously didn't teach all of them, but looking st that figure and presuming he teaches multiple classes, I'd say that it is perfectly possible he has been teaching for 3 years, and has expelled 154 students in that time.

(*) I'm from England, where we refer to each 'grade' of education a Year or Year Group (yr group). Sorry for any confusion