The best tutorial I've found on After Effects is Brian Maffit and Will Freeman's tutorial on 'How to create a hoverboard effect'. In it, they teach how to mask video layers out in After Effects, and how to animate that mask as well. For their tutorial, they use the example of masking out the wheels on a skateboard as it goes down a sidewalk, making it look like a hoverboard instead. Though this particular example might not be needed for most academic purposes, the general ability to mask a moving object in a video is very useful, especially when using animation as story telling, or even when trying to remove distracting elements from video. The video is also useful to learn key-frame animation and motion-tracing in general, which could be applied to images or cameras, too, not just masks. Maffit and Freeman teach the concept simply and clearly, making the short 5-minute video very easy to follow.