I was definitely always the excluded child at school - had 1 or 2 close friends at primary school, but when we tried to integrate with the bigger groups of girls we'd always be accepted very warily and sometimes pushed away. Always the last to be picked for games or anything when people were choosing teams (e.g. British Bulldog).
I always wondered why the teachers didn't pick up on this - seeing the same old faces left at the end when teams were being picked, why didn't they make someone like me team captain for a change and let me do the picking, just to make it fair? No, it was always the sporty kids who were team captain.
The worst moment was when I did a tennis club one summer holiday and there was an odd number of children taking part...so, rather than than mix things up so everyone got a chance to play against each other, everyone else was paired off to play against each other and I was told to practice my serve on my own and hit balls against a wall!
What a waste of my parents money sending me to tennis lessons where I didn't even get to play against anyone else.
...my abiding memory is not so much resenting the other children for being more popular, more feeling aggrieved that adults who were in a position to try and even things out a bit in favour of children like me, seemingly didn't bother. I can also remember my ballet teachers openly laughing at me in front of other children at how clumsy my dancing was (my mum sent me to ballet in a doomed attempt to improve my posture...)
Did it continue into adult life? I've done 3 long-term jobs in my working career, and 2 out of those 3 companies had a definite 'clique' that I wasn't in (people that hung out together at weekends, went to the pub after work, etc) but at my second job, we had a wonderful team in my dept where the whole team WAS the clique.
We all liked each other, went to the pub together, had in-jokes and team nicknames that no-one else in the company got, and some of those people are still my best friends. The good thing is that at the time we had it, we KNEW it was a good thing and that we were lucky to have it, we never took it for granted.
We had a really strong team spirit and a closeness that was way above anything I felt at school and I really feel those years with those people were the formative years that shaped me as an adult. Of course it didn't last, people left and the company ethos changed, but I still feel glad I had the experience of what it was like to bond with people in a way I never had at school.