It's hard for me to compare an American high school to a UK school because I've never been to the UK. In the US, after-school sports and activities are borderline mandatory; parents almost always put their kids into a sport or activity starting at a young age, and all kids are encouraged to participate and commit to a group. Throughout high school, those who you spend most of your out-of-school time with (e.g. your sports team, your club) become your social lifeline. This is kinda how cliques are created I think, at least at that age.

Cliques have been made even more prominent by social media. Posting an Instagram picture with your group of friends or tweeting at someone allows the world to know for sure who your friends are.

But then there are kids who quit their sport or whose parents never encouraged them to join a club. So after school while everyone is hanging out and getting to know one another, this kid is sitting at home napping or watching TV. When they go to school the next day, this kid is now no better friends with anyone because he spent all day at home watching tv and doing homework. At the same time, everyone else became better friends because they spent a few hours after school together joking around and practicing their sport or their instrument.

Cliques are kind of a cycle. If you don't nudge your way into one at a relatively young age, it's very hard to become accepted into one. The longer you wait to try, the harder it gets, and the more distant you become from any social group. I experienced both sides of this because I switched schools. It's a tough situation for a lot of kids, especially considering many people that age do not yet have a solid grasp on their identity.