WHAT MOVIE IS THIS?!?

So in the movie (it was on much music) there were a bunch of girls in a sorority and they hated fat people so they didn't let them in there sorority or use them one night a fat girl who's mom i think was an important died on them they tried to hid the body but they couldn't and then the police questioned they all blamed each other in the end they found out it was a fat girl who got skinny and they used her ( sorry for the spoiler like seriously it's really good watch it but first find the name) HELP ME FIND OUT!!!