Response to "辛苦了“

My school principal oftens says "辛苦了" to me after I have spent an evening teaching classes. I don't know how to answer. I want to say, "Not at all", but I don't know how to communicate that sentiment. A friend told me to just say, "谢谢". Is that right? I'm afraid it sounds like I'm agreeing that the evening of teaching was bitterly hard work for me, when, in fact, I usually enjoy teaching, even on the frustrating days.

How should I respond?