"The other thing that confuses me is the phrase "play with your graduating class" is used a lot. So what's with the playing up? Be a stud at 15u, be one of the cattle at 17u...why?"
Not sure exactly what side of this you are on is, but in general once you hit HS you should be playing with your graduating class. If your young for your class that may mean playing up. In general at the HS level playing at a different grade level will work to your disadvantage when it comes to recruiting. I can buy the argument, up until you hit HS, that you need to play with the best talent possible and if that means playing up then do it. But once you hit HS you are going to hamper your recruiting chances by playing with a different grade. Again, there are occasional exceptions to this but in general, stick with your graduating class.