Dan Waters, a member of the Academic Relations team based in Florida, has been working on an XNA version of Pong as a tutorial for beginners. Last week Dan posted a link to the source code for people who are interested in playing with it. Dan isn't making any claims of brilliance for it and I haven't had a chance to try it out myself but I'm guessing this might be a piece of code a bright student might enjoy improving on themselves. The tutorial Dan has writen is clear and easy to follow. So this looks like a good starter project to me.
Dan actually comments his code BTW. Brian Scarbeau who is planning on using this sample in his high school computer science game development course notes that this serves as a good example for students as well as being helpful for understanding the code. For teachers who want to intriduce XNA to students using this project (or even without it) Dan includes a PowerPoint presentation with the source code as well. And there are even speaker's notes with the slides!
Dan and his wife recently brought home their first child and I'm guessing that Dan's time is being occupied with things other than programming these days. So I image that he would be happy to see someone else do interesting things building on what he has so far. Go check it out and maybe set some students lose on it.