To whom it may concern:

At the conclusion of my contracted days, (DATE OF END OF CONTRACT), I resign from employment with XXX ISD. Thank you for the opportunity to serve the students of (school).

Sincerely,

(YOUR NAME)
(YOUR ADDRESS)
(YOUR PHONE NUMBER)

If you're on a standard 9 or 10 month contract, they aren't "paying you for those summer months" - they're dividing a 9 or 10 month paycheck by 12 (or 13 if you're a first year teacher!) months so you get paid all year. If there are ANY shenanigans about getting paid and you've worked until the last day of your contract, call a real lawyer, not a Legal Beagle.

That being said, if a job - any job, ever - tries to withhold a paycheck because you resigned, call the state and federal Departments of Labor and report them for wage theft. It's the most common form of theft in the United States, and you'll be entitled to damages. Best part is that the burden of proof is on the employer for showing that you unlawfully abandoned your contract and somehow weren't due that pay for those days.

If you work, you're entitled to the pay for that work. Period. Employers can't even withhold a paycheck for failure to return equipment - that's also against the law unless specifically authorized by law or by your permission. They have to seek other recourse for that.