Find and correct a mistake in each sentence

1. Children after 15 must go to school

2. This exercise is optional - you don't have to do it

3. If you break the rules, you'll get punished

4 You don't have to do extra work if it's difficult

5. Students don't have to wear a uniform in our school and so you can wear what you want

6. If you have to do it now, I'll help you

7. You mustn't drink and drive, it's illegal.

8. You mustn't open the car door when the car is moving

9. You shouldn't eat chocolate just before you go to bed because it isn't good for your teeth.