You need to cover two issues - the copyright on the information you receive, and the copyright on the information you provide.
In short, you want two statements, one that people submitting information assign copyright to you, and another that all information is copyrighted but permission is granted for free use.
A web search for "copyright notice "permission is granted"" should come up with enough for the second. "assign copyright" will do well for the first.
Given the description of your site so far, though, you will probably find that your users are actually not able to assign copyright to you, because it is probably owned by their schools. So, avoid going to court if at all possible, and consult a lawyer if it's really an issue.
If you *really* want to make things complicated and dig into licenses, you are looking for a "non-free" license. "Free as in freedom" implies that the people you license the content to are given the freedom to do largely what they want, often with the provision that they may not change the terms of the licensing. One of the freedoms you grant is generally the right to re-sell. Most anything Gnu provides will not satisfy you, because that's how they view "free" - people have the freedom to sell the things you give them, but the people they sell them to then have to be given the right to give them away. It's a tricky line that is easy for the person selling to mistake or forget to mention, and hard to enforce because they can hide the information behind a dollar sign and you'd have to pay to find out they're breaking the license.
http://opencontent.org/opl.shtml is probably a reasonable compromise, if they bother to print a book and are willing to release it as open content they can charge for it, but they can't charge for network-based access to the content or the content itself.
But, if the information you have is so valuable that people might want to buy it, why not let the people you give it to sell it to other interested parties? It might increase the dissimination of knowledge. You're asking people to give it to you for free in the first place, they can't really object to not being paid for it. In short, I highly recommend that you do enough to CYA but beyond that not worry about it, if people want to use the information you provide, great.