"They are not students."
他 is male while 她 is female (the radical 女 means woman). When referring to a mixed gender group the male form is used hence 他们. Technically 她们 can be used but it implies the group is all female.
Yeah it does but not really. In writing youre technically correct but not necessarily when speaking. The pronounciation for the male "they" (他) and the female "they" (她) [and the "it" (它) I think] is actually the same. The literal translation for "these girls are not students" is 这些女孩不是学生。
That "tā men" is for male third plural. The other "tā men" is for female third plural.
If that happens again remind yourself to send a message to Duolingo to correct the error
You want to say "is not." "bushi"
The sentence as you provided you have "bu" ("not"), but no "shi" ("is") after it.