Most of my school career was spent at one school, with a primary and secondary and sixth form all on one site. I think I started when I was 7, and left when I was 18. My core social group remained reasonably constant throughout that time, and only evolved very slowly, and school really became my life. All of my activities (Orchestra, cadets, rowing, etc) were school run activities (apart from a couple of years of Beavers/Cubs when I was younger, and going to church Sunday school for a number of years). Pretty much all the people I knew were from school for all that time, and I think I was encased in such a bubble that it never really seemed apparent to me that there was anything beyond the school life I knew...
I think about it a fair lot now, and I've been at uni for nearly a year (academic year that is)... I wish I could do it all again, but for different reasons - I feel like I worked reasonably hard and got good grades and stuff, but never really developed socially there, and never really did much outside school with my friends, and I don't remember an awful lot of it. If I could do it again, I would work less, play around more in lessons, make a much greater effort to be confident (I was ludicrously shy for most of my time at school), and also write a diary every day so I could remember more of it - I'm sure that every day something funny happened, and I wish I could remember all o those moments...
I feel the same - I was quite miserable a lot of the time at school, but there were so many times when I was just content, didn't have any worries, and everything just seemed constant, and I liked it that way. Since coming out, there has been so much more to worry about... Take me back! (Although there were many instances at the time I wished I could just leave school, in hindsight it was really great, for much of the time I really was genuinely happy - I never seem to appreciate things at the time).
A big gripe for me is that I think I could have been so much more socially successful, engineered a much better social group, and had much more fun if only I hadn't been so desperate to please everyone else and tag along, and had instead forged my own path and been much more confident... Opportunities lost.