After graduation, both Bakugo Katsuki and Izuku Midoriya became fighter jets pilots. They trained hard to be where they were. One day, both of them were sent on a mission, which ended up both successful and not. Successful—because they did the job. Unsuccessful—because Izuku Midoriya got heavily injured. He almost lost both arms after he tried to bail out of his jet after getting hit. He was no longer able to fly.

That's why he became a teacher. He was teaching young pilots while Bakugo Katsuki stayed in the sky.