My mom won't let me wear dresses/skirts?

I'm 13 (almost 14) and my mom won't let me wear dresses or skirts on any occasion! Not even for church. She says that I look like a sl*t even though the few skirts I have go down to my knees and my dresses are the same or like 2 inches above. It makes me so mad. Everytime we have school dances or my friend's parties everyone looks at me weird because I'm wearing shorts or pants and they're all dressed up. :[ How could I convince her to let me?