The Glockenspiel Beater : This weapon can be thrown with pin point accuracy to contact with a desired part of a victim's anatomy with almost no effort. It is one of the subtler weapons as it can easily be launched by a nonchalant flick of the wrist whilst the assailant casually stares in the other direction. Beater throwing is optimal good fun if the head of the beater can be removed and thrown independently from the stick thus giving a two-fold attack strategy.
The Coconut Shells : These are for your more tactical assailant. They can either be used for the basic 'trap someone's fingers in them as they slap shut at 47 mph' gag. OR, for the slightly more adventurous attacker, put one coconut shell over some unsuspecting victim's ear and hit with a beater (see above) until victim has perforated ear-drum.
The Xylophone Keys : Hard, metal, heavy, sharp. So many possibilities, so little time.
The Snare Drum Brush : Most popular of all. This tightly bound weapon consisting of half horse-hair and half wire is most effective when drawn agonisingly slowly over naked flesh - popular with the 'fat kid' bully network.

Maracas were the hand-grenades of the music room arsenal. Best launched from the upper platform in the drama studio, maracas would explode on contact with floor or head, scattering the enemy with small white pellets and imaginary gobbets of flaming napalm.

Sheet music: although not a regular weapon, at our school a boy was sent to hospital after the kid who was handing out sheet music decided to just throw it at the class, and cut the boy's eyeball open.

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.

Timpani Beater: During a fit of aimless classroom fidgeting I slid a timpani beater down into a cardboard tube so that its round head stopped it going all the way through. Mid-conversation with a slightly dim girl, I absent-mindedly flicked it towards her, and the drumstick shot out of the tube and hit her perfectly in the eye.

Fortunately I escaped punishment, as she screamed that I had hit her with a "trombone stick", and everyone was then too busy laughing for there to be any chance of repercussions.

hur hur "re-percussions"

Please write in if you spot any other unintentional punnage on the Law of the Playground. What a hoot!

The Guff Trumpet: Take one trumpet, guff with vigour into the horn end, and "play" it at other pupils. Be careful not to inhale.