There was always something clenched and anxious in me until the classes were over. Thus the late, great Seamus Heaney in an interview he did some years after retiring from active teaching at Harvard and other places. Beautifully expressed, typically so, yet so surprising, especially to someone like myself who is never more relaxed than when he's in a classroom or lecturing.
It wasn't always so, by the way. In my first year of teaching the classroom was a place in which I felt on trial. But since then it's been my second home. Not sure what happened to cause this, but I'm glad it did.