Ugh, do I remember the thigh gap thing. When I was a cheerleader, this was how my teammates gauged your worth to the squad: with your thighs together, you should have three gaps - ankle to calf, calf to knee and between your mid-thighs. We called it the "keyhole." Most girls had the first two, but only those of us with all three gaps were considered in shape and therefore deserving of our spots on the (very competitive) squad.
How I survived my teen years without developing anorexia is nothing short of a miracle. Unsurprisingly, I am still friends with zero of those girls :/