Many junior players do what I call "the grind." They attend large group lessons and get very little personal attention. Because the pro must keep the kids moving, they do a lot of hitting balls back and forth on the half-court, so that they can get the maximum usage of the court. That is 4 players on each court. Because of this, the players learn to be consistent, but boring. They do not learn how to hit angles, or attack the net or learn how to play the full court.
What is the problem? These kids get to a certain level of competition and get completely discouraged. They don't have the skills to compete at the higher level. Then what happens? They drop out.
In actuality, when I teach a player, I do not care about their immediate results. I am thinking about when they are older and what skills they need to enjoy tennis for a lifetime. These skills, fortunately, revolve more around doubles than singles. Look at the size of your club's doubles leagues versus singles leagues. No contest! So, the serve, volley, half-court volley, overheads, approach shots, serve and volley etc. become paramount. These skill are much more difficult to teach in a large group situation. That is why they are not taught these skills. The money is too easy and teaching the "fine skills" is too difficult. You can see why there is such a dropout rate.
I personally love all the calls I receive from ex-students who picked the game up again after college and go on to enjoy many years of fun and fitness through tennis. A junior ranking is great, but learning the "fine skills" of tennis is life long and priceless. Learn a skill.
Good luck on the courts!