they were jealous of Vanessa
She was not close to fat but very skinny and the girls were mad that the boy liked Vanessa instead of the other girl. Every time Vanessa tried to ask them what was wrong, they could not give her an answer but to do these awful things to her and then play innocent. It was pure jealousy. Especially when girls call another girl fat who is clearly skinny and rips on her hair and calls her slut when she was not slutting, pure sign of envy and seriously, jealously like that could turn deadly. I know this is only a movie but trust me this crap really happens. This is how school shootings happen too
Oh and after she almost died from overdosing on sleeping aids, because of them, they made fun of her calling her a druggie when she OD'ed on sleeping aids because of them. Suicidal because of them. If she would have died, they would have not had any empathy or regrets. But if Vanessa did something horrible to them after everything they put her through, Vanessa would be called a disgusting person or something and put away. You wonder why kids take the law into their own hands some times?