Thanks for the advice guys. I do take plenty of clothing for my long rides, though to bog myself down for a distance I could run in 10-15 mins if the worst cames to the worst does seem OTT along with all the school books.
As for what to wear to school, I like to wear what's comfortable to wear during the day as well as fitting in with all of my freinds, which is basically a hoodie with jeans or cargo shorts (if it's sunny and reasonably warm). When we're running round kicking a ball at break time, the hoodie usually comes off.
I also like stuff that's easy to move in.