I find that teens get restless with overheads because they equate it to classroom instruction. I also find it odd to arrive at a huge high school auditorium with a dinky little fold up screen to use for transparencies. Keep in mind for those to be seen, sometimes it is necessary to turn off the lights and then the audience can't see you as you speak nor can you see them. Additionally, low lights encourage sleepiness for some audiences. If you must use overhead transparencies, use them quickly and only when necessary rather than leaving the projector on throughout your speech.
Some people get stagefright due to a lack of self- confidence.
If you focus on WHAT you are speaking about . . . WHAT you are trying to communicate . . . Don't worry or think about your hair or clothes or trivial things. If you act like you have something of interest to share, your audience will be your partner in combating stagefright because you will be subliminally encouraging them to be interested. Think of this speaking engagement as a chance to eagerly share your views about organ donation and transplantation.