I am answering the OP's edited note : 'how to figure out the correct grammatical number to use'.
I suggest :
Scores below 40 earn a "study hard" comment.
The number (or even gender) of the students is actually irrelevant to the necessary grammar. There is no need for the possessive pronoun - or any pronouns at all.
And it fits for a single student, also.
The statement is about a logical connection between scores and commenting.
(I only answered this 'cos @Mari-Lou A made me do it.)