The piano, if left on castors, can serve many purposes. At its simplest, it can crush people against the wall, causing anything between a winding and organ damage. Or use it to barricade the music room door, for a session of uninterrupted violence.
Get it out into the corridor, and you can use it to smash open lockers, and batter down the doors of locked store rooms, allowing you to gather pots of glue, paint and detergent.
Get it outside... then you can sit on top of it, and roll into an adventure written by Roald Dahl. You might need the stuff you robbed from the store rooms to thwart the wicked headmaster.