Izuku's Class Schedule:
From 4:10 pm and until 5:00 pm, Izuku trains and tries to master his quirk, that is why he has more time than Jamie before he can leave school. Although the league of villains attacked class 1-A, the big 3 and the pro heroes were called to the scene so Izuku's quirk wasn't used much in the battle.
Both Jamie and Izuku's schedule are the same, but Jamie's hero lessons are practical lessons in which he can practice his quirk actively which is why he doesn't train as much as Izuku does.
A/N: Keep in mind that Izuku did not inherit All Might's Quirk. In this story, All Might was the one that saved Bakugo from the sludge incident and Izuku was not helping Bakugo, instead he was watching from the sidelines with Jamie as he does not yet know how to use his quirk very well and if he tried to help, he might as well have been the no.2 victim in the incident. All Might gave Mirio the OFA Quirk.