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, ...
Note that ‘score’ is originally from incising a mark - literally scoring a mark in the side of a tree, in order to count. One score is one mark. Multiple scores are multiple marks.
A ‘score’ is thus a singular item – for example, the total that a student gets in response to one test.
Scores–the plural–is a collection of results, either within one test, or ...
They get study hard if their SCORE (instead of SCORES) is below 40, right?
After reading this I would say to you, study hard
Take a step back; how would you describe giving a numerical mark? You state in your opening article I would give him a score of 40.
For a numerical mark your description is score of 40
Therefore for "The Comment" your description ...