We bought a backpack by JanSport (think its called the Student Bag or something) that sounds very similar to the one Yummiemummie describes - bought ours over the internet from play.com for £20. Its only a plain black one as there were no decent patterned ones except for tartan! DD walks to school and so needed something comfortable and strong enough to carry all her stuff in. It's not 'cool' to carry a rucksack but DD is not the type to style over comfort!
My DD also not concerned with what is 'in' and will actually try to avoid it, especially if she thinks it's impractical. She has a Roxy rucksack which is pretty trendy as rucksacks go. Got it in a sports shop. She has lugged it around full of folders etc for over a year now. It replaced a very similar one, same brand, and we only bought her another one because we needed a present for her and she never asks for anything. Much better for their backs than the silly canvas ones too!