It can be difficult to balance choice, motivation and expectations when young people feel stressed. This can be used in school or at home. Must is the bare minimum of what they can do. Should is what you would prefer them to do. Could is if they want to push themselves.
At home we would use this for tasks such as eating. e.g. you must have two slices of toast; you should have two slices of toast plus an orange juice; you could have two slices of toast plus an orange juice and an egg.
Or for getting ready
You must brush your teeth and wash your face.
You should also wash your body.
You could style your hair.
At school this could be used to break down activities. e.g.
You must complete 3 sentences.
You should complete 5 sentences.
You could complete 5 sentences and complete a wordsearch.