What many teenaged idiot girls do, especially after drinking. Thinking they were in control and could go as far as they wanted, safely, and no further. As a Catholic high school girl, myself, I didn’t know girls like that, but I’m certainly not surprised that they were - and are - around.
Although not an excuse for bad behavior, why would she go to a “party” at 15 where no adults were present?
Whoa!! Motive?