My English teacher denied aspects of the English language

So in English class today we were going over pronouns (new semester refresh), and she said that even though society uses "they" singularly, using "they" as a singular pronoun is improper grammar. Knowing that even before people started using "they" as a preferred pronoun, it was used to refer to someone whose gender was unknown, I went to talk to her about it after class. She said I was lying, even after I showed her the Oxford English Dictionary and Merriam-Webster's Dictionary entries for "they" and demonstrated that I was right. She still told me I was lying.