One possible version:

June 8, 20XX

Dear Sir,亲爱的先生

I'm sorry to tell you that I can't go to school today, I've caught a cold because I swam in the school's swimming pool yesterday.

我很抱歉地告诉你今天我不能去上学，因为我昨天在学校游泳池里游泳，我得了感冒。

This morning I had a headache. After taking my temperature, my mother found that I had a high fever, immediately she took me to see a doctor, who advised me to stay in bed for 2 days. Now I am writing to you to ask for 2 days' sick leave.

今天早上我头痛。走了我的体温后，我妈妈发现我发烧了，她马上带我去看医生，他建议我卧床2天。现在我写信给你要2天病假。

With best wishes致以最良好的祝愿

Your student,你的学生，

Tom汤姆