Tell people firmly and politely that you won’t have time or be able to do something at work. This could be your head of department, or it could even be your class. Both of them will respect you for telling the truth. Headteachers are not impressed by someone who just says “yes” all the time, they are just grateful that someone is willing to do extra work. Saying no to something can be hard but if you know you are going to have to say no then practise what you will say beforehand and stick to your guns. The first time you do it will be the hardest and you can always offer to help out on another occasion.