Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
This letter is to advise you that my [son/daughter/other], [name of student] has had to miss school from [date] to [date] due to illness.
Enclosed is a statement from [name of student]'s doctor, [name of doctor], verifying this information.
If you have any questions, or require additional information, please do not hesitate to contact me at [phone #].
Sincerely,