The tons of unnecessary things you learn in school while you don't learn some vital things. I mean, I've gotta learn the square roots of negative numbers with that imaginary number just called "I" but no one's gonna teach us how to manage the finances of a household? Pretty glad I got cooking class at school though.
Also the fact that your image barely changes during secondary school (we don't have stuff called highschool here, just 6 years elementary, 6 years secondary and optional college/uni after it). I was a pretty silent kid at the beginning of it, but got pretty outgoing cause of friends outside of school at the end of it. Yet, everyone still sees you as the silent one. So I basically developed this "I don't care about anyone or anything at this place" attitude during my last 2 years.
Never got much issues with bullying. I mean, I was quiet but by no means a pushover. Some tried since they somehow thought quiet meant insecure, but they quickly learned that they were better off leaving me alone and letting me do my own thing. College was awesome though, got a fresh start there, no one knew me and everyone was more mature, so I could finally be the outgoing self I actually was at school as well.