Perhaps you could answer with 「へええ、そんなにおひさしぶりじゃないとおもいますよ。」... cos don't you see your teacher on a regular basis? "ひさしぶり" is something you usually say if you haven't seen someone in ages.

Also, if you're ever stuck on how to reply to someone in Japanese, this will help:
How to speak fluent Japanese without saying a word
Part 2