Following is a letter written by Kailash to his Class Teacher, requesting him/her to accept his absence due to stomach pain. You can use the same format by simply replacing the name with yours and adding relevant details in respective places.
The Class Teacher,
Since I had a severe stomach pain I was not able to attend the classes on (Date 1 and Date 2). As I still have a subtle pain I will not be able to come to school today also. Kindly excuse my absence and grant me permission to attend the classes from tomorrow onwards.