Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [School Official],
This letter is to notify you that our family will be relocating to [location] on [date]. Therefore, [Child's name]'s last day in your school will be [date]. I want to thank you for the education your faculty have provided to my [son/daughter].
In order to have a smooth transition to [his/her] next school, I would prefer to have all of the necessary paperwork ready ahead of time. This includes [his/her] transcripts and [other relevant information requested by new school system]. I would be grateful for your assistance. Anything you can do to facilitate this process will be greatly appreciated.
I will contact you this week to discuss this matter further. Thank you, in advance, for your help.