Bullied guy gets beat-up to death

Saw this movie on the TV and looked way older than the year I saw it. There’s this high-school gang leaded by a girl who inflicts pain on others to get what she wants. An specific scene I remember is her sticking a knife under a guy’s fingernails. I also remember this gang beating up a dude and he reappeared in the world in a ghostly form. He thought he was dead and was in some kind of limbo, but later in the film, he finds out he’s still alive, only agonizing. He knows this thanks to a bird who gets injured in his bedroom window, appears in the ghostly form for a while and then dissappears.