Translation:Are they students?
Merely that a corresponding construction already exists in Chinese: "他们不是学生吗?" “Aren't they students?” has a different connotation in English than that of the simple "Are they students?", and the same applies to Chinese. If the assumption was that they are not students, you could say "他们不是学生吧。" (or in English: "They aren't students, right/are they?")