So, no fat and pure muscle, wanting to lose muscle? Full blown AIDS seems to be the best solution to a problem such as yours.
Step 1: contract a wasting disease. AIDS is a good choice, as is tuberculosis, cancer, or multiple sclerosis
1a.) confine yourself to a wheelchair to speed along the atrophy process
step 2: ?
step 3: profit! you are now the skinniest girl in town! man, everyone is going to be sooooo jealous of you rolling about in your motorized wheelchair, spindly legs and all.
ETA: dumb questions require dumb answers. Please tell me you understand the connection between muscles and activity? If you some how magically lost all your leg muscles, how do you expect to continue volleyball, soccer, and cheerleading? What exactly is the appeal of spindly atrophy legs? cause that's what you're asking for. You do also realize that muscle burns calories right? and without muscle, you just become fat. No, you don't have giant calves/thighs/whatever.....however you do have an exceptionally skewed perception of your body. Sounds like Body Dysmorphic Disorder, and you'd do well to get your head on straight before you attempt any sort of 'muscle loss'.