I second (third?) the Osprey backpack suggestions. I just recently bought an Osprey Flare backpack and opted for it over others because it is:

1) very lightweight and water-resistant.
2) It has a compartment for a laptop and fits my 15" Retina MBP perfectly.
3) The main compartment has plenty of space for books etc...
4) I got the "juicy orange" colour because it would be the most visible in dark conditions.
5) It has looped zipper pulls which make it easy to open even while wearing gloves.

http://www.ospreypacks.com/en/product/city__trail/flare