Ответ:

1.

Our school has no uniform.

have

We don't have to wear a uniform at our school.

2.

The rules say we must be at school by 8.

have

We have to be at school by 8.

3.

It's very important that you start doing some exercise.

must

You must start doing some exercise.

4.

For homework tonight, you can leave exercise 1, but do exercises 2 and 3.

have

For homework tonight, you have to do exercise 1, but do exercises 2 and 3.

5.

It's important that you don't eat any chocolate on this diet.

must

You must not eat any chocolate on this diet.