In "The Magicians", it has been established that there are 100 students, 20 per year, in 5 classes. We know that Fogg would rather skip a full year than having more or less than 20 students per class:
"There is a Twentieth, Melanie. We go through this every year, and we will empty every high school and middle school and juvenile detention center on the continent till we find him or her or it."
(The Magicians, "Brakebills")
Then, at some point, Quentin, Penny and Alice are considered to advance to second year, and this rule is brought up again:
He had a point. It was an immutable fact that there were always twenty students per class, no more and no fewer.
(The Magicians, "Snow")
Then, Quentin and Alice advance to Second Year, meaning that there are now 18 students in First Year and 22 in Second Year. Later,
Amanda Orloff is eaten by the Beast.
This brings down the number of students in Quentin's year to 21. In the aftermath of that event, a student of the First Year drops out (though we don't know if he might have returned later), bringing down the number in his class to 17.
However, when Quentin's class is flying to Brakebills South in his Fourth Year, it is mentioned that a) 10 people from the Fifth Year are joining his class, b) that each class flies to Brakebills South in two groups, and c) Quentin's group consists of 10 people from his year and 5 people from Fifth Year. This would mean that the other group would have to consist of 16 instead of 15 people, even though it is mentioned nowhere.
When Quentin finally graduates, it is mentioned twice that his class only consists of 19 people:
An eleven-course banquet was served in the dining hall. The nineteen graduating Fifth-Years regarded one another with awe, feeling lost and alone at the long, empty dining table.
And later in the same chapter:
The ladder took him and the other graduates straight down into a circular chamber large enough for all nineteen of them to stand upright in a circle, which they did.
(The Magicians, "Graduation")
What happened to the two missing graduates? Evidently, they did not move to the next class, otherwise this would contradict the chapter about Brakebills South. I also doubt that they dropped out, as leaving Brakebills voluntarily is almost unheard of before
the attack of the Beast
as described in the tale of Emily Greenstreet:
"Who's Emily Greenstreet?" Alice asked.
"Emily Greenstreet", Janet said grandly, savoring the rich, satisfying piece of gossip she was about to impart, "was the first person to leave Brakebills voluntarily in one hundred and fifty years."
(The Magicians, "Emily Greenstreet")
