How do I keep people from asking if I'm okay?

Well I don't exactly show much emotion and I'm not a very "peppy" "uppity" person, so when whenever I'm sitting alone reading or listening to music, there is a multitude of people that come over to me and ask me if I'm okay because "I look sad." How can I keep this from happening (I'm in high school by the way) without smiling all the time?