https://en.wikipedia.org/wiki/Singular_they

This use of singular they had emerged by the 14th century, about a century after the plural they.[4][5][2] It has been commonly employed in everyday English ever since and has gained currency in official contexts. Singular they has been criticised since the mid-18th century by prescriptive commentators who consider it an error.[6] Its continued use in modern standard English has become more common and formally accepted with the move toward gender-neutral language.[7][8] Though some early-21st-century style guides described it as colloquial and less appropriate in formal writing,[9][10] by 2020, most style guides accepted the singular they as a personal pronoun.[11][12][13][14]

Your teacher was just one of those purists and it was never something with strong consensus for being wrong.

Shakespear used they as a singular iirc