[Your letterhead, if desired; if not, your return address]
[Date of letter-month, day, and year]
[Recipient's first and last names]
[Street or P.O. box address]
[City, State ZIP code]
Dear [recipient's name]:
As I reflect back on this first half of the school year, I think of all the many ways my son has benefited from having you as his teacher. So I want to express my sincere appreciation to you for all that you do, in such a dedicated and tireless way. I know so much of your efforts can go unnoticed, but I want to let you know that I do notice and that I am so very grateful for your willingness to constantly go above and beyond.
Thank you again, on behalf of my son and me. We truly appreciate you more than you know.
[Sender's first and last names]