The dark behind the curtain - Gillian Cross

Colin Jackus doesn't want to be in the school play and is angry because he's been forced into it. But gradually he starts to realize that it's not just a play - the sinster story they are acting has its roots deep in cruel reality and despair.Strange things are happening among the cast of the play and misery is seeping through them.
So when the leading actor starts to take on the evil personality of the character he's supposed to be playing, Jackus decides to delve deeper and try and stop the disaster that threatens.