For the war/bass drum, rage spells come under enchantment. CG sounds right.
A grand piano feels like it should be LN - complicated machinery. As a school try abjuration, it stays put and may need defences.
In a book I read once a violin was made of human bone and cursed. It's a natural for NE and necromancy.
The electric guitar has to be evocation and sounds CN.
Trumpets get associated with archons which makes them LG. As they're used to herald arrivals, maybe conjuration?
If flutes are used in the Pied Piper sense then LE has to be their alignment. Since enchantment's taken, illusion could have related effects.
Accordions are evil by their very sound. CE is the worst kind of evil. Demons and so on are associated with transformation which corresponds to transmutation.
Saxophones are NG by a process of elimination, and it doesn't sound wrong for them. Divination is the last school, ditto.
Those are some awesome suggestions. I'm particularly liking the idea of the evil bone violin; clearly it's time to make some undead dance an Irish jig. The trumpet -> archon -> conjuration angle is terrific. At first I didn't see the Pied Piper effect as illusion, more enchantment, but if the images it conjures are alluring enough for people and animals to follow, it works too. The transmuting accordion is a weird idea, but it's also the instrument which transforms the most during use, so I can see it. The only instrument which isn't really inspiring is the NG divining saxophone, so I'll have to see if I can find a better instrument for that slot.
@Lady-J thanks for those ideas, I'll try to work them in to the alignment framework for each instrument too.
Assuming I go with those instrument-alignment combos, the tricky part now is to give them all goals and powers. I'll ponder that and report back when I have more ideas.