Translation:They are not students.
是 means more of like "are" in this case. Just like you would say 不要 to say "dont want" 不是 in this case would mean more along the lines of "they 'are' not students" . 不 is almost always paired with another word and acts almost like a negative prefix. The opposite of this sentence would be 他们是学生 (they are students), so it makes sense that just adding the 不 would change the meaning to mean the opposite.
Unless im mistaken, 们 is saved just for plurals of human nouns (我们，你们，学生们，同学们，etc.) You wouldnt say 毛们 or 桌子们. You can tell by the 人 radical that its meant to be for humans. Chinese dont really have plural versions or suffixes of words, we just say the number. For example: 两个毛，五个桌子