One possible version:
June 8, 20xx Dear Sir, 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. 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. With best wishes.
Your student,Tom
Dear Ms Yang,
I'm so sorry that I have got a cold.Because of the bad cold I can't go to school these days.As a result,I want to stay at home for days.Thank you.
ZangHua.
To whom it may concern,
This is the writing notice from Li Lei. I am sorry for the absence from the English Summer Camp today due to the illness caused carelessness of forgetting to take the umbrella with me when I went out in the rain yesterday on my way home. I will see the doctor today for I've had a cold and if I feel better tomorrow, I'll come for Summer Camp tomorrow.
Best Regards,
Li Lei
Dear Mr.,
I'm sorry to tell you that I caught a cold and I'm not feeling well now.I am adviced to stay in bed for two days and can't go to school then.But I think I'll get well soon and be back the day after tomorrow.
Yours sincerely,
Dear Sir,
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.
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.
With best wishes
Your student,
Tom
Dear Ms Yang,
I'm so sorry that I have got a cold.Because of the bad cold I can't go to school these days.As a result,I want to stay at home for days.Thank you.
ZangHua.
Dear Sir,
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.
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.
With best wishes
Your student,
Tom
Dear Mr.,
I'm sorry to tell you that I caught a cold and I'm not feeling well now.I am adviced to stay in bed for two days and can't go to school then.But I think I'll get well soon and be back the day after tomorrow.
Yours sincerely,
Dear Mr/Ms ...,
I'm not feeling very well today. I've got a bad cold. My father took me to see a doctor this morning, and the doctor looked me over carefully and gave me some medicine. He also advised me to stay in bed for a day. So I can't go to school today. I hope I'll get weel soon and go to school tomorrow morning. Thanks.
Your student
Dear Miss Chen,
I am ( ),I am so sorry to tell you that I have got a bad cold with headache,tiredness,inappetency and insomnia.My doctor advised me that I need a rest for 2 or 3days,so I ask for leave for 2 or 3days untill I am fine.
Thank you for your time and have a nice day.
7 July
Dear Mr. Smith,
On account of an attack of the flu with a severe headache, I shall be unable to attend classes today, for which I submit a certificate from the doctor. I shall be much obliged if I can get your permission.
Respectfully yours,
John Warner
Wang Li
尊敬的史密斯先生：
由于重感冒并伴随严重的头痛，我今天不能上课，特呈交医生证明。如获批准，不胜感激。
致
礼!
约翰·华纳
7月28日
Dear Ms Ma,
I am sorry to tell you that I can't go to school today. Last night my mother had a high fever and a bad cough. Perhaps she has caught flu. I will take my mother to Tiantan Hospital and then look after her at home, so I couldn't go to school today. So I would like to ask for one day's leave. I will be very grateful if you grant my leave.
Yours,
Li Li