Dear Tom,
I'm very happy to hear from you .
I am sorry to hear about your situation,and from your letter I know that you regret sleeping in the class.You want to behaviour better on your study an in school.Here I will give you some advice ,hoping these can help you get rid of it.
Firstly,you can listen to some light music to help you fall into asleep.Secondly,you should do some exercises,so you can keep healthy and a good mood .Last but not the least,you can review your lesson,and you will come across some questions,so that you can concentrate on your lesson.
Looking forward to your reply!
Yours,
XX
（上面写上你自己的名字）