Tribes by Catherine MacPhail
Kevin is determined that he'll never join a gang but his path crosses the Tribe's when he saves one of them from a rival gang. Invited to take their initiation test, Kevin plans to break the oath of secrecy and tell everyone. But he falls under the spell of the gang leader, Salom, and becomes a member. Kevin then discovers how hard it is to break away from the Tribe's rules and Salom's power, for when he's challenged Salom always makes you sorry. In this case he fastens on to Kevin's little sister, Glory, and Kevin is forced to take the initiation test again as his sister freezes with horror crossing a beam high above a ruined building.