Alice showed exceptional natural talent as well as she was top of the class. Penny took his exam test in record time and also seemed to be a natural. Other than Quentin's final Intro test he didn't seem to have any remarkable progress. The intro test was explained to him to be a well known first time magic use fluke. He didn't seem to be excelling in class at least not like he was used to before he got to Brakebills. So why was he even considered for advancement?
Warning! Dont read this answer if you havent finished the book as it pretty much gives away the ending, but answers the question.
I think he was considered because of his potential and
Because the Watchwoman (Jane Chatwin) was controlling the events of the book and influencing them as they were happening because it had to be HIM and his gang to defeat the Beast in Fillory. If Q didn't advance a year, he wouldn't have made friends with the Physical Kids like he did, wouldn't have ended up with Alice, and Penny wouldn't have ended up a hermit studying the Neitherlands.
Oghma's comment makes sense. I would also like to add that Quentin is an excellent pupil in his normal high school while exhibiting unhappiness and seemingly depressed most of the time. The story is mostly narrated around what Quentin sees and feels. It is quite possible that Quentin is a competent student of magic, on par with Penny and Alice, but he does not recognize this or does not give much credit to his own skills and talent.