The most common and most likely answer to this crossword clue is the 7 letter word INFANTS
I've seen this crossword clue in The Daily Telegraph
I
N
F
A
N
T
S
Buddy explains!
The straight part
We know that "First-class students" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case first-class students = infants.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.
What does INFANTS mean?
INFANTS noun- A very young human being, from birth to somewhere between six months and two years of age, needing almost constant care and/or attention.
- A minor.
- A noble or aristocratic youth.