Thank you! I have seen this movie a zillion times,and that part has always bugged me! I highly doubt that was the style in that town, since none of the other girls in the movie, (including extra's at the school) wore them like that. And if it was the style, they sure didn't have the right to make fun of the way Ren was dressed!
I assume you are talking about Ariel and Chuck in the scene where Ariel says to Chuck,"you'd never guess that your dad's a fire chief..." As she's saying this she is putting on her red boots. If so, in this scene it seems like she is a hurry to get going and maybe just tucked one pant leg in the boot and then realized she needed to hurry and didn't bother to take the time to tuck in the other one. I do this all the time when I am wearing my boots and in a hurry. So perhaps it's not a slip up, it's just human nature.
I also think that her pants and boots are that way because she is in a hurry since I am sure she is not supposed to be in the woods with her boyfriend in the first place!! As for not knowing who the girl and her boyfriend is, have you even watched the movie? It seems obvious......