One possible version：
Your letter reached me yesterday. It said in your letter that you were punished for using cellphones, which puzzled you.
There is no doubt that cellphones can bring us much convenience. We can keep in touch with friends and families wherever they are. Besides, the cellphone is a way to fun.
However, there are some problems with using cellphones. The ring of phones can disturb teachers and students in the class. It has been proved that often using cellphones does harm to health. Another problem is that some students can spend too much time and money on cellphones and sending messages, which will result in bad results in study.
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As a student, he should concentrate on study. It is right for school to prevent you from using cellphones.