Growing Up & Facts of Life, Nature 

This is the story of a strange and disturbing friendship seen through the eyes of Natalie as she gets to know Tulip Pierce, a delinquent girl most others go out of their way to avoid. Nobody wants Tulip in their gang. She bunks off school, is rude to the teachers and makes herself unpopular with her classmates by telling awful lies. But none of this matters to Natalie who finds Tulip's behaviour exciting and dangerous. At first she doesn't care that other people are upset by Tulip's bizarre games but as the games become increasingly dangerous and sinister, Natalie realises that Tulip is going too far. Way too far...Tulip becomes even more destructive and after a row with Natalie she commits a terrible crime. This is a compelling story in which Anne Fine explores the dark side of a friendship bordering on obsession, and sensitively depicts one girl's gradual decline into hostility and violence. - from Amzon