I agree with Coach May as well, although maybe not as strongly. I am of the belief that the curve MUST be taught properly. I myself am a victim of not having this done, and at age 35, I am paying for it to this day!
To add another perspective to this though, I believe the distance of the mound that you are playing with makes a difference. From a 45 ft. mound, the curve doesn't have that much time to break anyway, so a good changeup will be just as effective. Different story as the distance of the mound increases.
Finally, if you live in an area like I do, you will have outside influences to deal with as well. We have a baseball academy that kids go to that often teach things differently than what we do at the high school level. Then, it becomes a matter of how much the player (and his parents) have their trust and faith in you.
Good luck!!!