The sheer poison of girls in a single sex school.
Much happier in a co-ed when I finally moved, it was a totally different atmosphere much more mellow.
PE of course, freezing cold muga while the teachers were wrapped up warm with their tea flask, favouring highly the sporty kids.
The lack of options at GCSE level.. I wanted a trade I didn't want to learn what they were offering schools shouldn't be churning out exam passing clones they should be nurturing students strengths imho.
Tbh I left in year 10 and then as a grown up I've studied many course quite happily at college.