Maybe you should try ONE school, namely, yours. You could even prune it down further to certain majors or departments. If someone says "do you have it for English as well as Math/Computer Science?" then you know its gaining steam.

The idea of class reviews, teacher reviews, along with "do I need the book?" would be a natural progression as others have discussed.

To get more popularity with your own school, heavily brand it as long as they don't get on your back for using their logos. Because its a tool to facilitate their students they may not care, but because its a tool that could dampen their book sale profits, they might.

Another opportunity to make it easier is possibly integrate with existing school systems. I went to Virginia Tech, and for web developers inside the school system (again, speaking merely from my own experience) they had APIs that you could lock into course/class IDs, and even had an API that could authenticate with University PIDs and passwords. Doing so this way would automatically manage all of the classes and book information - then the students would merely have to thumb up or down if they found the book helpful. When you expand schools, integrating Facebook or other social networking credentials would be the obvious route, but to paraphrase 37Signals, "you don't have a scaling problem until you have a scaling problem."

You have an EXCELLENT idea - anyone who has been to college and has felt the weight of text book costs (along with the economy) would instantly see this as a great opportunity to save some beans.