One possible version:
June 8, 20XX
Dear Sir,親愛的先生
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.
我很抱歉地告訴你今天我不能去上學，因爲我昨天在學校游泳池裏游泳，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.
今天早上我頭痛。走了我的體溫後，我媽媽發現我發燒了，她馬上帶我去看醫生，他建議我臥牀2天。現在我寫信給你要2天病假。
With best wishes致以最良好的祝願
Your student,你的學生，
Tom湯姆