I just downloaded Green Screen App and I am in love. Chroma Key / Green Screen / Blue Screen programs have always been not that good or really expensive, but this is easy to use and will only cost you a few dollars in the App Store.
Here is a quick 20 second video that I made by taping a piece of green paper to the wall and grabbing a bit of Lego. I got the background image from Google and I added the lasers with Gun Movie FX (Another great special effects app).
The interface is really easy to use and you are able to layer three videos/images. Classroom applications are limited only by a students imagination. I can already see students adding themselves to historical footage in a history presentation.
Watch the video, download the app and give it a go 🙂