#students is a local temporary table I used instead of the permanent sample table students. Just remove the #.
g.SetSize is the calculated row count of the set in the partition (the GROUP BY group).
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden