I have found that having some stress in the classroom helps to keep the students focused on the topic and on track. I limit the amount of stress and the amount of time they are under it. The knowledge that there will be a quiz at the end of a lecture will usually help to keep their minds on the topic at hand. I remember back to when I was in College and how that knowledge always seemed to make the lecture that day more interesting and informative, and hopefully the same will apply to my students.
I do believe that too much stress is counter productive and must be avoided, but the necessary level is a fine line and unfortunately is easy to mis-judge.
