Should it be "everyone wearing a face mask" instead?

Could someone please help? Thanks.

It's correct as written. Realistically, it doesn't matter, English speakers will know what you are saying either way.

Thanks.

But I would like to know which is the correct version if I were taking an exam.

Face masks plural.

In regard to an ESL test, I think the typical understanding of the sentence would require the singular "a face mask," a single mask for a single individual.

The sentence, however, can be read as referring to every individual who has multiple face masks that they wear.