They should / shouldn't tidy the classroom before they go on holiday.
should
They mustn't / shouldn't leave the school next year - it's not a good idea.
shouldn't
They mustn't / don't have to stay at school in the afternoon - they can go home.
don't have to
They must/mustn't start studying for their exams this weekend.
must
You must / mustn't cheat in exam if you don't know the answer.
mustn't
You mustn't / don't have to finish this project today - you can do it tomorrow.
don't have to
What should / must I do in this situation? I'm not sure.
should
I think that they must/should talk to their teacher about the last exam.
should
You mustn't / shouldn't call your teacher by his first name - the school doesn't allow it.
mustn't
You should / shouldn't try to save some money if you want to buy a car.
should