All Might not telling Izuku about the whole "bones exploding" issue and not giving him One for All until right before the entrance exam was more than a prank - if Izuku had activated it out in the world instead of where Recovery Girl was close at hand, he could have crippled himself permanently or killed himself.
When we see the way the U.A. Staff treat their students, its no wonder All Might blackmailed his way into becoming a teacher. After all, they take bets on how many students will die, and Nezu is apparently a pedophile.
