Wouldn't it be more beneficial to be accelerated either way? If you're going bad, at least you get a practice run and can do it again the following year. If you don't like the subject, then you can get it over and done with quicker. If you like the subject, then good for you. I don't think there's any disadvantages to the person to stay per say, it's more so that they are recommended to drop to help the ranks of the other students that the person may affect.