Complex sentence: His favorite'school sport' is'locker-knocker time' where he locks Peter in his own locker which is come back to him in kind when Spider-Man convinces Flash to lock himself in his own locker whenever Midtown High is attacked by supervillains.
Split into simpler sentences: His favorite'school sport' is'locker-knocker time' where he locks Peter in his own locker. Karma comes back to him in kind when Flash locks himself in his own locker whenever Midtown High is attacked by supervillains.