The most common and most likely answer to this crossword clue is the 5 plus 7 letter word UPPER CLASSES
U
P
P
E
R
C
L
A
S
S
E
S
Buddy explains!
The straight part
We know that "Groups of pupils on top floor/They’re highly ranked" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case groups of pupils on top floor/they’re highly ranked = upper classes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.