I agree with pmsobx as long as you do it the right way you can put on a little more weight while staying healthy.
You are just perfect! Girls at your school are probably just jealous! First advise, don't worry what others think about you! You know you don't have a eating disorder and you don't have to prove it to anyone! Here is some good advice on gaining weight! GOOD LUCK!
Eating is the first and most important factor that is needed to gain weight. It's simple - you need to eat more calories than you burn. This does not mean eating anything and everything that is not nailed down. Carefully plan your meals with foods that will help you gain lean mass, not fat. I suggest 6 meals. Breakfast, lunch, dinner, and three other small meals scheduled throughout the day. The goal is to eat every 2.5 to 3 hours.
Focus on foods such as steak, chicken, fruit, milk, vegetables, cheese and assorted types of nuts. It's recommended to eat 4 - 6 pieces of fresh fruit a day, and at least four tall glasses of milk. Milk is a great, cheap protein source - take advantage of it. A sample snack meal would be a glass of milk, an apple, and a hand full of peanuts.
Take time to lay out a meal plan. What you eat is the most important ingredient in a successful weight gain program.
I got this information off of the internet. Keep us posted with your success!