Mom wont let me wear a thong?

My mom wont ley me wear a thong. In school i feel soooo uncomfortable with lines through m pants-.- i hate it. How dod i convince her? In 14 btw and im old enough. Im even a dancer and have a show soon and were not allowed to wear reg underwear for this one and she still wont let me! So m only choice is to wear nuthin underneath my costumes! Like really! Im ol enough!!! So please how can i conince her? Thanks guyyys(: