the difference between "au revoir" and "a bientôt" is quite subtle in my opinion, I usually say a bientôt when I am expecting to see the person i am talking to rather soon and "au revoir" when we will see each other at a further point in time or even when i don't really know when i will see the person again. I also usually use "au revoir" rather than "a bientôt" in more formal situations coming back to your teacher situation, maybe he just said it to be friendly, like some kind of way to show that he was willing to see you guys again. that's what I do when i say "a bientôt" to somebody who I know chances are, i'll never see again :) I hope it helped