One possible version:

Dear Mr Wang,

       I'm very sorry to tell you that I ean't come to school this morning. Could I ask three days' leave?

My mum is ill. She has caught a bad cold. She is in hospital now. And my father is away at the moment.

So I have to look after her.  I will come back to school soon and study hard all the lost lessons.

                                                                                                                          Your student,

                                                                                                                            Li Lei