How to write a letter to a board of education allowing someone else to enroll a student in school?

well I'm 16 and i live with my Nana and my mom needs to write a letter to the school people to let them know that they have my moms permission allow my Nana to enroll me in school. i just need some help on how to write it. i live in Georgia. i know i have to get it notarized and everything. like i said i just need to know how we would go about preparing or writing this letter.