User blog comment:Jkphantom9/RWBY clip on Lazer Team Livestream/@comment-119.74.232.100-20140624074018/@comment-10390252-20140624082252

They might legitimately be transfer students. There is no reason why Cinder could not have corrupted them but told them to stay at their academy because it is more useful to her plans for them to have a facade of legitimacy.

FWIW, I'm guessing that Mercury and Emerald are both seniors (about 20-21 years old) compared to our heroes who are all freshmen (17-18 years old except Ruby).