I saw this slip up the first time I watched this movie, so it's pretty obvious. When Josie goes into the bathroom at the high school after everyone laughs at her for having the LOSER stamp on her forehead, he hair is parted on the left side. During different shots, in the bathroom and coming out, the part in her hair changes sides.
When someone looks in the mirror you see everything opposite. At that part, she is looking in the mirror, so in this case, her part did not change sides.