Movie about criminal young girls

I was watching a trailer once and my partner was being so annoying I couldn't get to see much, but it was very interesting!

Unfortunately I only remember loose bits, where two young girls were sharing about some murders they committed or something like that. There was a scene where they were dancing in majorette style uniforms while a guy (or two guys?) were choking what I presume was their music teacher to death while they danced around him. Pretty weird.

It's in colour, and I think it's modern, perhaps from 2013/2015.

Extra info: the film seems to be set in the past, not sure the exact time but it didn't seem to be recent, some kind of boarding school setting. Uniforms maybe? wish I remembered more.