It’s a short story compilation, I think all by the same author. One story is about a cancer support group up in some mountain-y commune, and a child dies by accident at the end. Another story is about two best friends with one of their boyfriends. He’s kind of awful, and I think there’s a scene where the three of them are at a roller rink. Another story is from the second person, and it’s about a frumpy redhead who is too skinny for ballet? At the end she gets to ride bikes with this boy.
I know this is a long description. The cover is of a forest, with a few kids on a rope swing jumping in a pool.
It’s amazingly written. I read it like 8ish years ago so it was published before that
I think some of these are in Melissa Pritchard’s book ‘The Instinct for bliss: short stories”
It’s searchable by word, here:
https://archive.org/details/instinctforbliss0000prit/page/n9/mode/2up?q=Ballet+thin+bikes