Liposuction is probably the most effective way to lose 20 pounds in two weeks--which, of course, is not safe for ANYONE, let alone a 13-year-old girl. At the age of 13, you shouldn't even be considering losing 20 pounds in such a quick amount of time--no one should. And even if you tried losing weight on one of those gimmicky "48-hour Hollywood Diet," you'd primarily be losing water weight anyway--NOT FAT.
Effective weight loss should be done the smart, SAFE way--through a balanced, nutritious diet and exercise. You may not be able to fit into that dress in two weeks...but safe weight loss over the long haul should be what you go after.