I used to, then I became a scuba instructor and had to teach all the time. The nervousness will lessen the better you know your subject matter. The first few classes I did, I was really nervous, then it got much easier.
I also used to speak at schools all the time for "career day" thingies for paramedicine. That was easy, because I really knew what I was talking about after being a medic for several years. It came naturally while speaking to the students because I was so confident with my knowledge of the subject.
The best thing to do is REALLY know your subject, then you will be much more at ease.