Sophie, I am still thinking about our common problem of gaining weight when our knees are hurting and keeping us from moving much. One of my doctors told me that for every pound that I could lose, my knee would feel a loss of 6 poounds. At first, I didn't believe her, but I tried it and yes, my knee hurt a little less. I don't believe in special diet for weight loss in adults, much less for young people, so what I did which may work for you is to just eat a sensible , well balanced diet, only cut the volume of it just a little. For instance, if you usually grabbed a handful of cookies and it was 4 of them, then only choose 3 of them the next time. If you like certain entrees for your main meals, just cut the amount by a couple of spoonfuls. Over 2 or 3 weeks, you could lose a pound. Believe me, it works. Do not leave off any food groups. Eat widely, differenet colors, etc., but just a tiny bit less.
Also, the first doctor I found who helped me a little gave me in addtion to a shot , just one exercise for mal tracked patella... just lie on your back in bed and lift your stiff leg up and lower it very slowly , almost to the mattress, but not touching it, and then raise it up slowly again. He said 40 times ! (I know!! At first I did less). You will feel this in your abdomen too, but it will strengthen the muscles on the sides of your leg to help keep your knee cap from moving around where it will hurt. I don't believe this could hurt anything else that is going on, but you will know when you try it, if it could do some good.
I sense that you may not be able for several reasons to access help with your problem, so I am trying to help you in practical ways to get through it until you are a little older and have more options. A good knee brace is another thing to help when you have stairs to climb or heavy loads to lift, like a backpack of school books. You know the kind, that has an opening for the patella and fits real snug around the joint. My brace has steel bars on each side and is heavy and cumbersome, but I love it because it helps my knee hurt less. I don't wear it all the itme, of course, as you don't want our own muscles to forget how to work and support you. You can purchase the elastic support kind in most pharmacies.
Hang in there!!! Find some joy in each day. It is a beautiful world if you look in the right places.
Keep reading the forum. There are some wonderful people on line who freely share what they know.
Hugs to a brave girl........