"Which seat should I take?"
While one might argue that the chorus to this song is a mere bit of insipid pop pablum (that she didn't even have the gumption to pen herself, mind) one might look at it as an (admittedly oversimplified) illustration of her lamentations on morality:
Even in the structure of the song, she's at the bus stop specifically waiting for a bus, when her friends show up. Already temptation has presented itself; she's made the decision to forgo what is expected of her: the uniform mundanity of her week, as illustrated by the designated places she is required to be (she catches the Bus at the Bus stop. This is axiom the rest of the week, hence why there's no songs about it)
But here we have the paradigm shift: The arrival of her friends. And she is presented with two simple choices: Front seat or back. The back seat has the more obvious open seat, and that is what the backseat represents: easy, path of least resistance; be a passenger, be passive, surrender control of your destiny for simple comfort. What might she have to do to sit in the front seat? She'd have to ask someone to move, she'd have to assume control of something (in teenage driving heriarchies, the front seat passenger is usually saddled with responsibility; generally operating the stereo, reading a map, or 'am i clear on that side?' but responsibility nonetheless.) These responsibilities increase tenfold were she to commandeer the driver's seat. "Which seat should I take?" Indeed.
The most telling factor of all is that this question remains unanswered. Choosing her destiny is not what she signed up for this day; her focus on this Fri-ee-day (the most treasured of days for teenagers, best loved for combining the shirking of responsibilities, a day that starts with the rigidity of school and boasts an evening that lay open-ended) is simple pleasures: partying, fun, looking forward to the weekend. One might argue that in the video they show her in the back seat of the convertible standing up with her awkwardly dancing friends, but the surrounding imagery of her being surrounded by admirers, and the only real voice (the rapper is isolated and otherworldly, like a queer Greek chorus) is hers. By this point in the video she's redefined her role beyond her choice of seat.
What's even more interesting is that her real life has come to reflect this: Her Good Morning America interview, and her reactions to her newfound fame all speak of a young girl from whom the public demands an answer, demanding her to shoulder her destiny and take umbrage and shame at the notoriety that fate has presented her with, (Ark Music Factory even offered to remove her from their site to alleviate all the negative attention, but she politely refused) but she rejects it. She politely reminds us all that she is but a 13 year old girl, who wants to party and have fun, and has no interest in answering to us, or anyone, as to which seat she will take.