I've read Horikoshi regrets some of the more 'extreme' things he wrote Bakugou doing to Izuku in the early chapters (particularly the suicide baiting). So he's probably doing something that we see in the American comics MHA is a love letter to: pretending something that happened in the early story just didn't. Though in the tie-in story for the first MHA movie, we see a panel of Izuku getting beaten on by other kids, Bakugou at front and center.
Of course, All Might has a history of being terrible at reading people. He honestly thought that Endeavor was the guy to go to for teaching kids, and didn't realize the guy all but hated him.