Good Morning,
I would suggest you write it from your heart but keep it simple and straight to the point. Try not to repeat yourself and state the facts you want to share with this person.
This is a very nice idea and I am sure your teacher will be touched to recieve this letter. Also a nice card would work with a special verse that may suit the situation.
You might start your letter with "This brings sincere thanks..........."
Good Luck,
Kitt