They do, I remember being one. Some are pretty blatant about it, like the 14yo girl that talked about sex all the damn time. She'd find ways to bring it up in pretty much any conversation. Then suddenly one day she stopped talking about it and we all knew she'd lost her virginity and discovered it wasn't as incredible as she'd fantasized it being.
Most of us are a lot more discreet than that, but preteen and teenage girls get just as horny and delusional as preteen and teenage boys.