What is iMovie green screen effect?
If you shoot a video with a single colored background (often green or blue), iMovie allows you to make that color transparent. You can replace the background with any other video clip, graphic or still image as you like. That is the green screen effect in iMovie.
The most common example might be as a TV Weather Forecaster who walks in front of a Map of the US and explains the upcoming weather events.

With iMovie green screen effect, it's easy to superimpose anything into any shot. For example, you can transport yourself to anywhere you like, or appear in your favorite movies. That’s interesting, right? How to use green screen effect in iMovie? Many people may be bothered by it. Now follow the steps to apply green screen effect in iMovie and you will find adding iMovie green screen effect is just a piece of cake.

How to apply green screen effect in iMovie 09 or 11

Step 1. Start a new project

From File > Import > Movies to load videos to iMovie from your Mac. Please make sure your videos are supported by iMovie. If not, iMovie will not allow you to edit the videos in it and you may need a Video Converter application to convert the videos to iMovie MP4 so that you can load them to iMovie for editing. Drag videos from the Event browser to the Project Library.
iMovie green screen
iMovie green screen
Tips: If you want to edit your favorite DVD movies and make yourself appear in the movies by using green screen effect in iMovie, you should use Handbrake open source software to rip the videos first since iMovie doesn't support DVD.

Step 2. Enable green screen function in iMovie

You may be confused when you want to use the green screen function in iMovie because it never shows up in the menu. Well, you need to turn on the function first. Go to iMovie -> Preferences, and select “Show Advanced Tools” to enable iMovie green screen effect.
iMovie green screen effect
iMovie green screen effect

Step 3. Apply green screen effect in iMovie

Select a part of video clip you want to overlay on your background clip and drag it to the start place of the green effect.
green screen in iMovie
green screen in iMovie

When you drag the video clip on another in the Project Library, a small window will pop up instantly. Hit Green Screen to apply green screen effect in iMovie.
iMovie green screen tutorial
iMovie green screen tutorial

Step 4. Adjust iMovie green screen effect

In the viewing window, you can drag the four corners to change the frames of the clip. Hit preview button to see the result. When it is all right, click the Done button to finish iMovie green screen effect adjusting.
iMovie green screen problems
iMovie green screen problems
Done!
Now you can enjoy your videos!

Your students can be anyplace anytime through the use of movie magic!

I found some interesting green Screen Clips and tips on Youtube which might be of interest like this one...