Gossip Girl is, I am convinced, a criminally underrated show, and I will defend it to the death, but it is not a show purports to have a realistic plot. There's basically nothing that happens on this show that would ever happen in real life. And yet I am about to way overthink one of these nonsensical events, and yes, I realize this is somewhat arbitrary, but this one has stuck with me for some reason.
Here's what happened: Chuck, a 19-year-old orphaned billionaire (I know, I know), realized before his girlfriend Blair did that she wouldn't be happy as a student at NYU (not that we've ever seen a sign of her going to a class or anything, anyway), and so got her into Columbia as a transfer. The show states this pretty baldly, and then moves on, and never offers any explanation of how Chuck went about this particular bit of magic.
The way it's phrased - "Chuck applied for you," I think, although I don't have a transcript on hand - suggests that Chuck actually went through the normal application process in a fraudulent way. I suppose he could do this, but it seems awfully complicated. Transcripts and recommendations, no problem. And he could get someone to write essays. But did he hire an impersonator for the interview? (It's not like he hasn't hired Blair impersonators before, but going to a college interview is on rather a different scale than making Blair jealous at a party.) Since Blair is a current NYU student, I'm sure Columbia would want to do an interview - it's not like she's in Alaska and can't make it to the city. And wouldn't they want to contact her parents? What about financial stuff? Not that Chuck couldn't just write a check, but could he get her parents' tax returns and whatever else? (Yeah, he probably could. Never mind.) So: actually going through the application process is something Chuck could probably manage, but it seems extremely complicated and time-consuming. And it just doesn't sound like a Chuck scheme.
The way Chuck actually accomplishes things on this show is generally by staring at things intently, and occasionally saying "I'm Chuck Bass." In this case, "I'm Chuck Bass" was probably followed by "And look at my billions of dollars." Is there now a Bass (or Waldorf?) building being constructed somewhere at Columbia? I mean, why not? He has ridiculous amounts of money and only maybe five things in the world he cares about at all*, so it seems completely in character for him to call up some business contact who's on the board at Columbia, point out that their endowment has undoubtedly taken a hit recently, and offer to remedy that if only they'll let in this one random undergrad. This explanation seems far more likely.
Of course, in the context of the show, it really doesn't matter what means Chuck used. Because the point isn't that Chuck could accomplish this. The bigger point here is that Chuck can easily, almost unthinkingly, give Blair everything she wants, but the difficult part is for Blair to recognize and accept this. On one level, this is highly problematic: it suggests that, to be happy, Blair must surrender her own agency and let her boyfriend give her what she needs. And this level certainly exists on the show, but "Women, Power, and Money on Gossip Girl" is another post for another day.
This particular instance is more complicated than that, because Gossip Girl essentially sees Blair and Chuck as one person, in a literary sense. By accepting Chuck's help, Blair isn't just giving in to him, but to herself. Chuck is the only person in Blair's life who loves her as she is; when she is around anyone else, she puts on an act, and/or the other people ask her to change. Blair herself spends much of the show not really wanting to be Blair, but even when that happens, Chuck still wants Blair to be Blair.
So this really isn't about admissions officers or interviews or schools at all. It's about Blair taking a step toward growing up, and growing into herself. Sure, Chuck knew before Blair did that she'd rather be at Columbia than NYU, but Blair's real issue is that she has to look past her own games and lies and recognize what her true self (represented by Chuck) already knows. Once she understands what she wants, the mechanics don't matter - it all falls into place.
* Blair, his business career, Nate, Lily, and maybe Serena, in approximately that order.