"They are not teachers."
们 indicates it's plural! This applies for both 他 and 她! If the gender is unknown or mixed, use always 他.
Also, in other comments they said if its a "negative" sentence 是 is not needed..
I had the same thought when I saw the options available for this translation
I'm confused, as there is no way to know if the sentence is for plural he or she...
That is correct, and in the same way, when a chinese says "tamen" you have to know from what is spoken if they mean men or women.
们 (Men) is used to make plurals.
她们 (Tāmen) = They (She plural)
他们 (Tāmen) = They (He plural)
我们 (Wǒmen) = We
你们 (Nǐmen) = You (plural)