Welcome to the forum.
It is great to meet you.
The first thing to say is that there are always idiots and bullies that you will meet who will pick on other people for a vast array of reasons - and I'm afraid that alongside other stupid reasons like being short or a different colour or having a different accent - many of us know that weight is something that they can focus on...
They are wrong when they do it...
It is good that you want to lose weight - as it would not only mean that the bullies could no longer focus on your size - but it would also mean that you would be healthier. It might well solve your knee problem - or at least make it so that your knees do not keep on getting worse.
If you start to eat healthily and become more active and keep it up - you will lose that extra weight.
If you do it now while you are still young there is a good chance that you will not have extra skin either...
No-one knows about loose skin... It depends on a lot of things like amount of weight lost, age when you lost the weight, genetics... It is not fully predictable...
I have a lot of loose skin - but I was a lot bigger than you and a lot older when I lost weight. I can say that when I wear my jeans and a t-shirt no-one would imagine that I have lots of loose skin and it doesnt stop me dancing as much (or more than) people who are half my age. Loose skin is nothing to worry about... It is a lot better than having skin filled out with fat.
I have only come back to the forum quite recently - but spent a long time here a few years ago when I lost a lot of weight... One of my friends then lost a lot of weight in her teens (a lot more than you have to lose) and I distinctly remember her telling me that she had no loose skin at all. I think that this was mainly because of her age... Her name was Invariant.
She hasnt been to the forum in quite a while - but I am linking page 1 of her old diary so that you can see the photos of her to get inspiration. She did that and got no loose skin.
http://weight-loss.fitness.com/threads/19373-My-Story
I know that you have knee problems - but think that you would benefit from starting to do some walking every day. The great thing about walking is that it takes no equipment and you do not look like you are doing exercise - so if the bullies see you there is nothing to make them want to start teasing you. You can even slot it into your daily routine so that you walk for instance to school.
If you wear a pedometer you can let it challenge you to walk more steps and therefore build up your stamina and exercise level.
Eating healthily is very important too.
Cutting out junk food and takeaways is a great first step.
Drinking 1 ounce of water for every 2 pounds that you weigh and eating more vegetables are both great too.
Bit by bit you can improve your food plan so that it gets healthier and healthier.
You may have to work on portion sizes too - but things like undressed salad are so low in calories that you should be able to eat lots of it without eating too many calories.
I suggest that you read the following thread - it may help you settle in and make friends
http://weight-loss.fitness.com/threads/57955-My-advice-to-newcomers
Good luck with your project.
