So what is green-screen?

Essentially it’s using software or an app to combine different images or video so that one appears as if part of the other. The green screen part makes use of chroma-key feature of the app so that anyone standing in front of a green screen will appear in the video with the green screen not seen at all, but replaced by another chosen background image or video. Wikipedia gives a much fuller explanation here: https://en.wikipedia.org/wiki/Chroma_key.

But how do I create a film in the classroom with green screen?

With an iPad, and a green screen app (such as the Doink green screen iPad app) and with anything in the background which is green (whether frieze paper on the wall or green sheeting, or specially made green screen fabric on a frame) you are ready to go.
Have a look at the Sway below for examples of videos created by pupils using the DoInk green screen iPad app, along with how-to guides to starting to use it yourself. And then it’s creativity on the part of your learners in the classroom – for further inspiration have a look at the DoInk green screen app blog  to see how others have shared about using green screen film-making in the classroom.