Here are 3 examples that you can use for resignation letter as Assistant Professor -. Resignation letters should use the formal greeting: "Dear".The first sentence in the letter should state that you are resigning and when. If you've decided to move on from your current role, then make sure to use Resume. Io's new tool to write the perfect resignation letter. Under the Texas Education Code, contract employees are required to resign no later than 45 days before the first day of instruction for the school year. I am writing to formally tender my resignation from my position as an Assistant Professor at Crestwood High School, effective June 20, 2053. Sample Resignation Letters.