First of all, YAY for allowing your students to be Form CREATORS!!! How cool is it that you are empowering them to reinforce their own knowledge in such a way that they get to learn something new while they practice what they've already learned?!
Second, I'll bet with just a tiny bit more time, one of those smart little muchkins will figure this out for themselves. Just so you have a heads-up for when it happens, I'll show you what they will discover:
While they're editing, they will need to click on the wheel/cog/settings to change their Forms to quizzes. At the top of the window that opens when they click the cog, they'll see three "tabs:" General, Presentation, and Quizzes. If they click "quizzes," then they'll be able to turn that on, making their form a quiz. They will have a couple of options to choose from... but I am sure it'll be pretty obvious what they need to do from there.