Hi, it's hard to say what's going on without more info, but there are a lot of people in in the same situation. I'm sure you'll here from some of them
Just some questions...how tall are you? Maybe you're at where you should be? Have you tried tracking your food to see if there is anything you may have missed? Fitday is a good tool for that. How about your water consumption? It should be at least half your body weight. How much salt/fat/sugar do you eat either in a shaker or in processed foods? What type of program if any are you on?
How much exercise do you do? Being in high school should actually provide you with a number of options not limitations. Can you walk to school? Is there a track you can walk or run? How about a gym/pool/weightroom/tennis courts? Can you and your friends go dancing/skateboarding/skating/
hiking?
Have you been sick/injured at all? Sometimes your body will hold on when it needs to recuperate from something, stress included. And on that topic, when was the last time you had a check up? There are a number of issues that may make it harder to lose.
Just some ideas to think about. Whatever you do, don't get discouraged. Just remember, healthy habits should last a life time. You don't need to be in any hurry. Just do the right things and the weightloss will follow.
Best of luck to you!