Ok, who is telling you that you are fat and why? Who decided that you have "fat legs"?
"The people around you" don't sound like they care about you that much, I tell you what why don't you just go out in a great big tent of a skirt and show off your skinny top half because that'll make them all a lot happier? I'll tell you why not, because that would be daft.
Not as daft as this though. I'll tell you what less than 1,000 calories a day will do to you over a prolonged period: you'll lose your sense of taste, you will find swallowing things difficult, you will be constantly dizzy and unable to actually do any excercise as suggested above, you'll become obsessive about food and spend hours working out how many calories there are in everything, and as for your friends, well you'll lose interest in them and won't have the energy enough to go out and see them, but quite frankly they don't sound like a good bunch of peope to be around.
Ditch them and the crazy idea that that you need to lose a stone and a half and get on with your life.
Here's a thing, I first became anorexic when I was 16 and started obsessing over how many calories there are in everything. I've wasted so much brain space to knowing how manay calories are in things. You name any item of food, any chocolate bar, any brand of crisps and I'll tell you how many calories there are in it. I should've been learning about things that would help me in life, instead I can tell you the fat content of a hobnob without thinking twice about it. Any I don't care more! It's just in there, when other people know about useful stuff. I don't even want to know it but it's a complete natural reflex for me to read every bit of writing on a food packet and I have to stop myself doing it.
Just don't eat junk, don't overeat and don't sit on your bum all day and you won't ever get fat!!!!