A resignation letter is the last impression that you leave behind, so it is just as important as the first impression you make.
In such a letter be professional, say thank you, express your gratitude, and be respectful. Don't forget to send the resignation letter at least two weeks before resigning.
A good resignation letter can make you look professional and respected, and it can also assure a good reference for other jobs in the future. A badly written resignation letter on the other hand can have a negative effect later.
Since you will probably need a reference later from the person you are writing to, and a good one at that, make sure you keep the tone very positive. This is the most important about a letter of resignation.
You don't need to include a reason when you are resigning, but if the reason is relocating or starting up learning again, then including a reason is OK. If you are unhappy with a colleague or the payment, it is better not to mention anything of the kind.
Offer your help in the letter. Even if it won't be accepted, it will be appreaciated.
Dear Mr. Atkinson,
I am informing you that I wish to resign from my current position as the English teacher of your school. My last day of work will be the 11th of May.
I would like to express my gratitude for all the years that I have spent being your employee. It was a pleasure to be a part of your team. I appreciate the attention and the time that you gave me during these years. At your school I have learned how to be a good teacher and how to organize all kinds of activities. These experiences will be very helpful in my future career.
Please acknowledge this letter of resignation. I will make sure to complete all my projects by the time of the resignation. Please contact me if I can help you with anything in the future regarding my previous job. I feel lucky that I have worked for your school and I wish you success for your future.
Secondary School English teacher