Here is a sample format for an Excuse Letter. This is often used when a child misses his or her classes due to an illness.
(Insert School Name)
(Insert School Address)
Dear Sir/Madame (Insert Teacher's Name):
Please excuse my son/daughter (Insert your child's name) from his/her classes last (Insert Date of Absence or Period) as he/she was ill. Kindly extend to her/him any assistance on the lessons or assignments missed during that period. Please refer to the attached medical certificate. (Include this only if he/she has one)
Hoping for your consideration and understanding. Thank you very much.
(Insert Your Signature)
(Insert Your Full Name)