There is some misinformaion out there about prep schools. Players cannot raise their core GPA (for qualificatin purposes) after having graduated from high school, so attending prep school does not help in that way. They can raise their SATs, though, so that is one reason to prep. Injury concerns, like McCoy's, are another. Sometimes kids are immature or need to put on weight, or just don't have the options they want. It is the GPA thing that has most people confused.
People also need to realize that there are a couple different kinds of prep schools. There are athletic factories and there are college preparatory schools. They are all lumped under the same heading, but the difference is very real. College preparatory schools are very rigorous academically. If you get a kid out of one of those, there will typically not be any academic issues because it is MUCH harder to get into than the University of Minnesota (or most colleges, for that matter). These schools are, primarily, in New England and cost around $40,000 to attend.
Milford, Hargrave, Fork Union, and a bunch of awful schools in North Carolina are different. They will take anyone.