Timing
Much of animation is similar to the rest of filmmaking: you need a narrative, composition, lighting, editing, a soundtrack to make a good film. However, one of the main differences in animation is the timing.
The basics
At its most simple: to create slower movements you need more frames of animation, for fast action you get away with less.
Students should be encouraged to understand speed, as you often find they go from one extreme to another. Upon realising their first animation appears too fast, they may start to create very slow sequences. It's a very tricky balance, especially when you are not too sure yourself, but it's heartbreaking when a student painstakingly creates beautiful but endless slow animation, for a rocket, or a sprinting hare!
Drawn animation is particularly good for understanding and learning about timing, as students can add new drawings (helpfully called 'inbetweens') in between existing drawings to slow action down, or remove drawings to speed action up.
Understanding filming on 3s, 2s & 1s
Ones
Traditional British TV and movies work at 25/24 frames per second (fps) so you could create 25 new frames for each second of screen time. This is called 'animating on ones'. However, it is very laborious and students should NOT be encouraged to do it.
Twos
Much commercial animation is filmed on twos, which looks nice and smooth. (This is a similar effect to selecting 12fps on some animation software)
Threes
Fortunately animation still looks great if you film on threes (or even fours) (This is similar to selecting 8fps on software)
Less
Of course, you can get away with fewer frames per second (animating on sixes, etc) but after threes action can appear jerky and things that should be smooth (like a ball flying through the air) can look unacceptable.
Especially with drawn animation, students will often declare "That was quick! Can't we play it back a bit slower?". And indeed you can slow the playback down – for example if you wish to slow animation recorded on threes, say to half speed - however this doesn't really slow it down - what it does is turns their animation into sixes, and work usually becomes quite jumpy.
With older students you should get them to understand that 'jumpy' drawn animation can be fleshed out with 'inbetweens.' With other types of animation jerky movement can only be solved by working more slowly.
You will often find impatient students filming the same drawing twice or more - again to try and 'slow down' their work. This causes the same problems - however, there is a positive spin on this 'mistake' - they have subconsciously grasped some of the fundamentals of animation timing.
For most younger students it is not essential they understand the difference between animating on twos or threes etc. What is important is that they understand how many pictures make up one second of screen time.