Very mixed.
I was privately educated up until I was 16 and I have some really bad memories of my secondary school years. The worst ones are being stalked and being physically assaulted (having a chair thrown at me and having my arm twisted up behind my back). I was bullied by a lot of different people from different friend groups. It was hell. There was very little that was done to help me. I think schools are different now with their non tolerance for bullying. I suffered a lot with my mental health and depression, including self harming, and this was back before people understood mental illness. My dad could also be abusive, so I didn't have any safe space.
I changed to a state 6th form when I was 16 and it was like a breath of fresh air. My mum said it was the first time I'd ever come home from school with a smile on my face.
However, I made lifelong friends at my secondary school, so believe it or not despite all the awful experiences, I do have some very good memories of that time too. The friends I used to stay with for sleepovers, in lovely houses with normal non abusive parents, and I'm beyond grateful that none of them ever questioned why I never reciprocated the invitations.
I stayed in the same city I grew up in. It's been 20 years since I left and earlier this year I took my children to an event nearby and we walked down the road and I showed them where I went to school. It was the first time I remembered it fondly, after spending so long trying to forget it and actively avoiding the area.
I got over it of course, but it took a long time. I became more resilient as I got older, with much less tolerance to bullshit. It definitely wasn't just me who was the problem either. I maintained a close circle of friends from that school and haven't had issues making and keeping friends since. I have marked out a reasonable career for myself and have always been spoken of well at work.
There are a few posts on this thread that minimise bad school experiences, which I've come across a lot of since leaving school. Usually people who shrug and say oh that's not a big deal, etc, tend to have had a much easier ride at school. I think that's a mistake. Bad experiences at school, especially bullying situations, can really affect someone's self esteem, and therefore the choices they make, for some years after. I still think a lot about my time at school, but fortunately I mostly remember the good things about it now. They are formative years, and bad experiences during those years should not be dismissed.