When you decide to leave your teaching position, to formally let your employer know that you're leaving, you'll usually need to write a resignation letter.
If the letter is being sent as a hard copy, then it should be in the proper business format, otherwise, if the letter is being emailed, it should have an appropriate subject line and proper grammar.
Though I have enjoyed the past [Number of Years] years with [School Name], I have decided it is time to move on. I have recently accepted another teaching position with another school. My resignation date will be [Date].
This decision was quite hard. At this time, I need to relocate to be closer to my sick mother-in-law.
I will be sure to complete all grading and associated responsibilities before departing this position. Please let me know what further steps or duties must be completed before my departure.
Thank you for all of the support you and the school community have given me. I will never forget your generosity and the important skills I have learned at [School Name]. I will forever cherish the time I spent with my students, colleagues, and this community.
I hope to stay in touch with you and our community. Thank you again for all of your support.