Yes, you're thin, but so are many other girls your age. When I was 13 I was also fairly thin, and I didn't gain much weight until a few years later. A lot of girls at my school were also very thin - not because they had eating disorders or dieted, but just because that's how their bodies were built! Some of them gained a bit of weight in high school, and some didn't. It's just how our bodies work!
If you're really concerned about it, the best person to ask if a doctor or a nurse at school. Please don't let the comments left on here determine your outlook on your body. Try not to let somebody judge you based on a physical characteristic. Most people realize how terrible overweight people feel when they are picked on, but they don't realize how much their comments about thinner people can hurt.