if your not used to carrying a backpack then leaving the whole backpad behind is a bigger risk then damaging it. its so easy to get up and leave without even thinking of it.
The biggest problem i have seen with mac airs and other laptops with heavy books is the keys leaving marks on the screen from the weight.
A lot of my lecture notes are on Blackboard as well but I don't always print them. I keep printed ones in my locker and I never bring the books in. I always read them at home or in the library.
Carbon-fibre case with ballistic padding. JK, just put it in any laptop bag or backpack and should be fine, just keep the cords in a separate compartment so it doesn't scratches the aluminium finish or crack your screen the worst.