>>280556793
Here's the original Japanese text
And this is exactly the spoken Japanese in the anime adaptation
Word for word, it literally says "Naganohara-teacher for a while draw cannot was said." "Hand injury ended up was said."
Not only can Japanese not be translated 1:1, it's also very context sensitive. You have to fill in the blanks yourself, but they are always obvious based on context. However, gender is not specified at all here. The character, Naganohara (which is a surname) has not yet been revealed to be Mio.
So, the English translator has to add the blanks to form a grammatically correct sentence. English isn't grammatically correct without the subject of a sentence being re-stated with pronouns. So what pronoun should be used here to refer to the mysterious and unrevealed Naganohara-sensei? "They" seems most appropriate. "She" is revealing more information to the English reader than a Japanese reader would have got at this point. But it's a minor issue.
It surprises me how many people think "they" is an LGBT-owned word. If you don't want to learn Japanese, fine, but at least understand English.