I tried not to scrutinize that scene too hard because you can justify her decision by saying she was drunk, scared of getting in trouble, and a teenager but it really feels like there was a simple solution to her problem. She could have simply called the police and reported the sign was down, without mentioning that it was her that knocked it over. She even mentioned that she would call when she got home, but she had a cell phone, why not just call right then and there, give the address, and bounce
But again young, drunk, and nervous so I let it slide.