However I think comments like "its more than most teenagers are up for" or "Gotta impress onto them that Budo is all about hardwork and determination man. If they think they can cheat their way in they will" are really unhelpful and in fact disrespectful to the children.
Unhelpful how? Disrespectful how? If they're up for it, they're up for it: the class will work for them and they'll be able to train and get something out of it. And if not, then not.