First, cut a frame out of paper. I usually pick rectangles because I'm just not that great with scissors. With rectangles you can get several different sizes of frames out of one piece of paper. Then, cut some contact paper slightly smaller than your rectangle. Next, stick the contact paper to the frame. Last, put the contact paper backing back on the frame to keep it from sticking to things before you're ready to use it.
We set out all kinds of things for kids to stick to the contact paper. Tissue paper and glitter are no-fail options. Since you can immediately stick these up in your windows, you can let the kids experiment with how much stuff they can stick before their art won't stick to the window anymore.
As you let the kids work, save the backing paper. If you don't plan on using the frames to decorate at school, when the kids are done use the saved backing paper to protect their art for the trip home. Once home, parents can take off the backing to stick it to their own windows or to back the frame with coordinating paper for display elsewhere.