Being in Math Rocks is like getting onto a Select sports team.
If you want to compete for the top in the AMC tests, then some work outside of class is important, especially in Middle School. Some parents push their kids and others are self-motivated. It may be that your son becomes competitive, or he may not. Or he may go through phases.
The later classes in harder topics will be often frustrating. But sticking with it will bring emotional and intellectual growth and a true sense of accomplishment.
The exposure to problem solving techniques and the advanced math treatment will have an impact on their intellectual maturity far beyond anything else. And this qualitative outlook will stay with them no matter what they choose to do.
