Group training is not always a good idea since some people may not even understand the jutsu, yet they won't speak up since they don't want to hold up everyone else. Another issue would be if someone doesn't reply as often will hold the rest of the group up which isn't really fair.
If senpais and senseis focus on newer members then the members who have almost mastered everything will have to wait longer which in the long term may make the situation a lot worse since they are the people who will be the future senseis.
Despite saying this, I understand your point, and I actually take newer students who've learned nothing as compared to someone who has 1/2 elements done already, but I'm just stating it can't always be done like that.