Spanx underwear for 15 year old?

I'm a size 10-12. I'm going out soon so i was going to wear my bodycon skirt and a black leotard..it looks gorgeous on me, really slimming but when i bend over or sit down there's that awful little roll :/ like if i'm going to be sitting on some boy snogging him i don't want to be concious about that roll...would spanx flatten my tummy a bit? What's the best spanx...obvs don't want the one that goes down to your knees cause the skirt's really short!! Thanks in advance :)