PE- the teachers, the clothes, the ridiculousness, the way it seemed designed to put you off sport.
Collective punishment. Teachers who engage in it seemed weak twats to me then, even more so now.
Pettiness about uniforms and makeup and hair. This has only got worse. I cannot dredge up any professional respect for someone who thinks it is essential to a child's education that they look a certain way.
Teachers being too fucking scared to challenge the psychopaths in training kicking the shit out of people at one end of the playground so pretending they hadn't seen it and focussing their ire on less formidable pupils doing appalling things like drinking a can of pop instead.