As others have said, you are not eating enough.
Leaving aside the health risks, eating too little IS WORSE for weightloss than eating that bit extra. After a period of eating too little, your body goes in to starvation mode, and although you are eating so little you wont lose weight. Your body thinks that you are in a famine and its trying to save your life, so it will hold on to every bit it can. Then when you eat more again, you'll stack back on the weight quickly.
The best thing for fast weightloss is good healthy food (no junk or sugary stuff) and exercise. You will be thinner doing that in a few months time then you would be eating 800 calories a day. Not to mention a LOT happier.
In regards to how fast you can tell the difference, pretty quick really. One or 2 kilos probably doesnt sound like much to you, but you go lift 2 kgs of anything and its a decent amount of weight
It depends on your starting weight as well - if you are quite overweight a few kgs might not be as significant on your frame as if you were quite slim already. On the other hand, the most healthy your current weight, generally the longer it takes to lose that 2kgs.
If you are concerned about school and bullying, is there someone you can speak to about it specifically? Remember, NEARLY EVERYONE gets bullied at school, not matter what they weigh, and finding ways to conteract on it is an essential part of growing up no matter what you weigh as a teenager.