Yep, definitely address the letter to their supervisor/line managers. The best thing(-ish, amongst others) that happened to me at work last year was discovering that my line manager had received a glowing email from a teacher I support, and it had been printed out and put in my file with HR. That is such a brilliant thing to have, for anyone. Also, doughnuts/a homemade cake and personal/individual letters/cards of thanks would probably go down a treat if you have the time, finances and inclination!