I have a 20 L Karrimor "Spike" bag, it's rugged and can take a beating, and as far as I am aware doesn't attract wierd looks (but then I'm at a strange college!), it fits A4 files but is quite small and i'll probably by a new one for next year, it does tho have loads of useful pockets I keep coming across, for instance there is one which fits exactly my supertool 200, and another one my Inova X5, and also a key clip. All in all a reliable bag but too small for [censored] loads of work books.

Other suitable makes are Lowe-Alpine 15,20 and 30 L bags, or Berghaus, which are notably comfortable in the new range.

-Bushman