I always wonder whether the people who complain about cliques and them sticking together aver think that when they too arrive to find a group of people they naturally look for the people they already know and like to go and chat to.
How many people arrive and think "oh I know A and B over there and we always have a good laugh together, but I don't know C so I'll go and chat to them..."
That's all it is really.
I went through 3 dc and various schools between them.
I made some mum friends, but would never have described me as being in their close friend circle except perhaps one, as I had friends locally outside school. Never found it a problem if I wanted to chat finding someone to chat to - and I'm not the most sociable person around. I don't think there was every a group outside school I'd have avoided because they wouldn't have talked to me if I'd gone and joined them.
In all that time I'd have perhaps described one group as a "clique" but it was basically a group of 3 cousins (there was a fourth cousin same year but they weren't really in the same friendship group) with a couple of their neighbours, and they were very close knit and always did things together after school and at the weekend.
However they'd always say hello, and by the top end of juniors ds was very friendly with one of them and has stayed friendly through seniors and leaving school. If I see any of them out now they're as happy to stop and chat as anyone.
I think the definition someone gave upthread of "A clique is a friendship group that I'm not in" is often true.
Now I don't say cliques never happen - simply that they're far less common than people think. They're in the outsiders' heads rather than really there.