A healthy dose of nervousness can heighten adrenaline and increase positive challenge. Too much nervousness, of course, also increases threat, which -- as you may recall -- causes the brain to downshift, which is not useful to you as a teacher/presenter/facilitator. As such, it is ciritcal to find a healthy balance.
First of all, consider why you are nervous. Are you worried about failure? Of what? not enough knowledge? (Don't worry, if you have your instructor rating, you will
Remember that the vast majority of your students will arrive at Ground School highly motivated. They want to learn what you have to teach. The challenge of course is teaching in such a way that meets their learning needs.
An added "complication" is that adult learners come to your classroom with many years of school experiences, not all of them positive. For those who have developed a negative schema around school and teachers, attending ground school may resurface some of these negative emotions and cause students to behave less than their best. Your development of a safe, engaging and inviting learning environment is critical. Karen Hume and several other great authors offer some excellent advice on how to do so. Do a little research, or chat with a more experienced and well-like GS teacher to get some tips. Consider shadowing another teacher to observe what she does well, and think about what you might emulate or do differently.
At the end of the day, perhaps the most important thing is to be authentic. Be genuinely enthusiastic about sharing your expertise and passion for aviation and your particular topic. Be open to new learning, ie. if you don't know something, don't make it up, but rather, jot it down and promise to get back to the student(s) -- and then do follow up.
Be prepared and have a plan for the beginning, middle and end of the class. How will you hook people into your topic at the beginning? How will you engage different learners (auditory, visual, kinesthetic) during the lengthy middle of the class? What learning activities can you build in that will allow you to engage the limbic (social) system and incorporate movement into your class? Be prepared, but don't be afraid to take risks and try something new. Unlike piloting, which insists you keep risky behavious in check, teaching requires some risk taking. As Wayne Gretzky says, "One hundred percent of the shots you don't take don't go in!" Read an article about teaching -- even if it is totally unrelated to GS -- and try out a new instructional strategy in your ground school class. And let your students know you are trying out something new and are not sure how it is going to work; they might respect you for it.
Nervousness can be alleviated, but it may never go away entirely. I have been teaching classes and facilitating workshops of of 3 - 500 participants for over a decade, and although I love it to bits and feel fairly confident in my knowledge base and abilities as a teacher, I still get nervous beforehand. Every time.