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Aizawa has expelled 154 students before he got 1-A. Assuming that he doesn't expel the recommendation students (long shot, I know), he's been teaching for nine years, and that's assuming he's expelled nearly every student he's got. Including recommendation students in the expulsion ceremonies, he's been teaching for eight years, still assuming he's expelled nearly every student under him.
Now, I can't believe that there were that many "unworthy" kids in each previous iteration of 1-A, so he's likely been teaching probably closer to twelve years to account for that many expulsions - aka pretty much right out of high school himself, since he’s thirty at the start of canon.
Aizawa. Honey. Please. Take a break. You need sleep.