It is common for high school students to have a boyfriend or girlfriend. But if you don't have one, that's okay. It is also common NOT to have a boyfriend or a girlfriend. Many times, if you have a boyfriend or girlfriend in high school, your relationship does not last a long time. And like Nigrescent Design said, it is a relationship for the experience. I had one friend in college, though, who dated his high school girlfriend for four years!
Do you have to have a girlfriend? Nope! :)