Diana says "sorry" to the girl trying to get by her to go out into the hallway, "exactly" what Maureen said when she blocked the gym teacher in the doorway to the locker room where Diana was smoking at the start of the movie.
Those are the first two scenes in the film, yet they essentially bookend the time-line of the story; which begins in the locker room and ends in the rest room. The edit jumps you forward from the locker room incident to this scene months later outside the rest room. During the actual passage of those months Diana matured and developed a conscience, which the author essentially credits to her friendship with Maureen.
Since the film's friendship dynamic is about both girls giving something of themselves to each other, I think it was just a simple or symbolic way to illustrate the extent to which that process had completed each of them by that point. Maureen protected Diana in the locker room and Diana would decide to protect Maureen in the rest room.
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