I hated all of it, with the exception of the academic learning. I didn't attend much - skipped almost all of year 8 and most of year 10/11.
Hideous uniform that I'd be teased on the bus for as I didn't go to school in my local area. PE, including tiny PE skirt with gym knickers (but your knees couldn't be visible when you had your regular uniform skirt on...). Art. Any technology subject that required carting lots of stuff around all day. Assemblies - although I preferred the religious kind to the kind where everyone got a horrible lecture because of the bad behaviour of a minority. Homework - I actually did my best in class and didn't muck about, but I really resented the idea that I was expected to do more at home and was never free of the thought of school. So homework rarely got done. I hated the noise and crowding in corridors, and could never eat lunch at school because of overwhelming noise in the canteen.
Mostly I hated being treated 'like a child' and all the stupid petty rules that had to be followed for no reason other than that they were rules. I hated all the condescension. I couldn't wait to leave and go to work. Already at home I did most of the housework and childcare for younger siblings, and I was more than ready to be part of adult life. My problem was that I was shy and so instead of speaking up about unfair and petty things I'd just harbour a quiet and growing resentment.
On the plus side, it's turned me into a libertarian-minded individual with no tolerance for petty 'because it's the rules' types, and in my own work with children and young people I've always done my best to treat them as individuals.