At a young age Izuku Midoriya was kidnapped by the league of villains after having his parents killed before his eyes. Before his parents died, his mom told him to become the number one hero. When he was taken to the villain hideout along with other quirkless children, they were all put under experiments and torture for years.
Years later, Izuku is 15 now but he still holds his ideals to become a hero, to save people from the pain of losing loved ones and being put under torture for the fun of the sadistic villains.
There are vast differences, UA high school doesn't start with hero courses, it's just like a regular highschool that qualifies for the heroes in training to go to UA University and learn to be hero. Izuku Midoriya will start late in UA High school.
I don't the characters or BnHA, I just own this one singular concept and my attempt at a fanfiction.