A 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.

As a student, he should concentrate on study. It is right for school to prevent you from using cellphones.

题目分析：本文属于半开放性提纲式书信体，英语写作是一种创作性的学习过程。启动知识信息储存，构思立意，谋篇布局，遣词造句，对语言表达的正确性和准确性、思维的逻辑性和文章的条理性都比口语要求更高。先审题，弄清写作要求，审题是写好作文的前提，理清思路，确定体裁、框架结构和内容。提醒考生注意时态及语态的使用。

【亮点说明】文中使用了非常好的短语和句子为文章增色不少，比如There is no doubt that cellphones can bring us much convenience., does harm to health, It said in your letter that you were punished for using cellphones, which puzzled you. It is right for school to prevent you from using cellphones还运用了which引导的定语从句等等。并注意了句子的衔接如： However；Besides等等。