Greenscreening is also known as chroma keying or compositing. It is the practice of using a green (or blue) screen to apply visual effects. There are two programs on the iPads that allow teachers to use this - Touchcast Studio and Greenscreen by Do Ink. There is no free resource on the chromebook for this technology. There is a paid program called WeVideo that will create green screening. While there is a free version of WeVideo, the free version doesn't include that capability. If you have a Mac computer you can use iMovie for free.
Below is a hyperdoc on greenscreening and below that are some videos on how to use the different programs.