Interview Season

I have been invited to interview at a high school for a teaching position. I have been asked to prepare a 10-15 minute demo lesson for  about 15 8th graders. First reaction-freaking out. I have never had to teach for only 15 minutes. What do I teach? Where do I begin? I do not know what level these students are on or what subject I should teach. I guess I will have to figure it out though. I can do this. This is my passion and my light will shine.