Dear {Principal},

I would like to take this opportunity to express my gratitude to you. Though we see each other every day, I never told you how much I appreciate everything you've done for our school.

In the past {number of} years that I've been a teacher at {school}, I have witnessed firsthand how dedicated you are to helping our students, teachers, and the entire school community. {Cite a couple of specific examples.}

You're a great leader and an inspiration to many, including myself. Thank you for always looking out for our school's best interests.

Kind regards,
{Your Name}