Dear (teacher),
I wanted to wish you a happy teachers day. I have had an incredible time in your class and I just wanted to let you know that you are an amazing teacher. You have made me feel loved and excepted, and you have helped me find a love for (insert subject they teach). I could only hope to ever have a teacher as good as you again. I hope that you have a spectacular day, today.
Sincerely,
(Your name)