This is my resignation letter several years ago. Some people may tell you to be as short as possible. I kind of think that's cold, especially if you are wanting to leave on good terms. Everyone's different.

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With this letter, I respectfully resign my position as ___________ for __________ ISD following the completion of my current contract.

I have thoroughly enjoyed my ______ years of employment in __________ and I am thankful for the support that I was granted from my students, colleagues, and administration. My time teaching in __________ ISD will always be remembered fondly and much of my growth as an educator will be attributed to this district as well.
Thank you for the opportunity to work for ______________ ISD.

Sincerely,

__________