Definitely buy or build a second battery pack if you're not going to charge at school. The stock SLA's will not last long if you drain them until the bike won't go any more. It sounds like you're using 12-15 Wh/mile, so doubling the Wh capacity would work nicely for your 12 mile round trip.
If you don't mind spending a few hundred bucks, a NiMH pack like this would probably double your useable capacity, due to Mr. Peukert robbing some of your lead.
http://www.batteryspace.com/index.as...ROD&ProdID=995
And at 11.8lbs, it's probably lighter than your current pack
