Do you carry a laptop with you? Im also a teacher and I started commuting much like you are now. Backpack and all. I found the weight on my back, and the added pressure it was putting on my bum quite annoying. I eventually moved to a rack and bought myself one of these bags, much like the previous poster mentioned. Its an Arkel commuter pannier and converts to a messenger bag when not in use. It is expensive, but I am also carrying a laptop, so it was worth the added cost.

http://www.arkel-od.com/panniers/com...asp?fl=1&site=


Now that I have a rack, i'll never go back to a backpack..the rack is much easier to deal with and I have found no ill side effects with it on. With two inexpensive pannier you can, (as the poster above mentioned) carry a lot of groceries.