How do you write a letter to school principal for leave due to sister marriage?

These letters actually do not need to be overly formal. The school just needs something to show the child is out with the parent's consent.

Dear Mrs. Doe,

Please excuse Timmy Smith (student number 00000) from his classes on August 10th and 11th. We will be out of town due to his sister's wedding. If possible, could we please gather his school work ahead of time? That would be appreciated. If you have any questions, please contact me at (000) 000-0000 before Friday.

Thanks,

Jane Smith