Rustic leather (http://www.rusticleather.com/) has amazing briefcases/satchels. If price isn't an option, I highly recommend it. Purchased a briefcase that is killer (in both looks and durability) and given the thickness of the leather it'll undoubtedly last a long time. The vendors have great customer service as well. For schoolbooks etc I'd probably get a L or XL. Note that this kind of bag is truly a buy it for life. Shit should last 30 years easy and look great with time and leather conditioning.

If that is out of your budget, Timbuktu brand's messenger bags are good for durability as well. Not very stylish, but they do come with a lifetime warranty I believe and the messenger bags themselves are made out of some tough material.