This is hard for English speakers to learn as although we use different tones, they are not 'set' and certainly do not completely change the meaning of a word. We mainly use them to add what you might call the emotional meaning of a word beyond its literal meaning. well there are some cases where we change the meaning and perhaps just pointing some examples out would help your pupils.
For example
What are you doing that for? Can be said in a way that means a genuine question, or an annoyed/exasperated/upset/frustrated/angry statement, all depending on the tones used.
I have some contact with a tonal language that has 3 tones that completely change the meaning of the words. People are taught to think of 'doh ray me' and whether to use a doh tone, a ray tone or a me tone for each part of the word. You could extend this to doh ray me far. These are names of notes in the Western tradition of music that most people will know (certainly if they've ever seen the film The Sound of Music!). This might help get sufficient difference to the tones, but they might not match the correct Chinese tones, I don't know. It would help you get the point across, anyway. You can also draw a line representing the shape of the tones, rather than numbers, so a word that starts with a low tone, goes up to a high tone, then returns to the low tone would be accompanied by a line that looks like the general symbol shape for mountain, for example. Up to a peak, then down again.