I totally do that already... not to mention regular setup and repair of the school guitars, paying for spare parts out of my own pocket.
I'm doing up my fretless (refinish, new pickups, the works) and aiming to have my 5string fretless finished for by time I leave 6th form... I'm planning on donating them both to the music department along with one of my old 6 string jacksons (I know some metal kids in the first years have taken a fancy to it, even though it's a bit too awesome for my liking).
I think giving is a good thing, especially for the cause of music. some people just need it more than we do at the end of the day.
if you aren't willing to give your instrument away, then you can always just lend one that isn't your main player to someone who wants to learn, and pick it up for gigs or practices when you need it. just to prove I'm not just being hypocritical, that's where the other one of my jackson 6 strings is, and that's where my stingray is.
the real cure for GAS is to give something to someone else.
