Here's the question: is the idea so good, that you'll never have another idea that good? Is this billions and billions of dollars material?

If it is, tell them you want to do another project with them for the class and see how you work together -- if all goes well, then you can work on the first, more exciting project.

If it isn't -- it's just a good idea -- then hang out with them for a bit, work with them on a small project, and go for it for the class. If it breaks up, it was a good experience.