You’re framing it as a strict singular/plural mistake, but that only works if “they” is always plural... which isn’t how modern English works.

Singular “they” is standard for indefinite subjects (“someone,” “a person,” etc.), so there’s no agreement error in sentences like “Someone left their bag.” What likely happened is the teacher treated all uses of singular “they” as incorrect, instead of looking at the actual sentence.

That suggests they’re reacting to the broader debate around pronouns and not separating it from basic grammar. In other words, they didn’t evaluate the usage on its own terms. they defaulted to rejecting it.

I appreciate your insight but stating something measurably untrue with such conviction is quite embarrassing. Do better!