I went back to my high-school to borrow a lab coat for a Halloween party and while I was there I ran into a few old teachers and had a chance to thank them and have a good conversation with them. While a lot of kids 'hate' their teachers I seemed to be the near opposite.

Also, I recommend doing this for other things too, for example, I recently had an operation on my hand and the nurses and other staff at the hospital were awesome. I'm going back for a follow-up appointment and I'm going to bring a thank-you card and a couple of flowers to give to the ward.