Denise -- I'm hesitant to suggest an online site that's free, as the quality varies highly. Does your school allow you access to student standarized test scores from previous years? That might be a good place to start -- those scores are usually broken down by topic, which might be particularly helpful when creating cooperative groups for different activities.
I agree that there is a definite need for something like this and easyCBM.com looks pretty good from what I can see. The downfall is that it is not free and our district is not looking to spend any money right now. Does anyone know of a free program that offers the same thing?