Burn for Burn by Jenny Han and Siobhan Vivian

Kat is sick and tired of being bullied by her former best friend. Lilia has always looked out for her little sister, so when she discovers that one of her guy friends has been secretly hooking up with her, she’s going to put a stop to it. Mary is perpetually haunted by a traumatic event from years past, and the boy who’s responsible has yet to get what’s coming to him.

None of the girls can act on their revenge fantasies alone without being suspected. But together…anything is possible.