grammar - Singular they = general they

Well, here is the situation.
I have four students. They did a test. Suppose that the regulation is a student getting a score below 40 gets the remark "study hard" which is written at the bottom of the work instead of a numerical score. When I want to confirm this, may I say:

They get study hard if their SCORE IS (instead of SCORES ARE) below 40, right?

Answer

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.

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(I only answered this 'cos @Mari-Lou A made me do it.)