Obviously not the only reason, almost definitely not the main one, but certainly a big one I think. I have siblings a good couple decades younger than me, at 11 my sister is 5’7 (apparently the lower end of avg. height in her year), 14 y/o brother 6’4, both overweight and massive. I’m 5’3.
I do find it hard to imagine how teachers would be able to break up fights, or even properly confront bad behaviour if a pupil that big is known to be aggressive. Surely this must impact?