It's a public high school. If you're asking how involved are the parents, the answer is very.Should read "How good are the parents?". High Schools could easily be measured by the percent of parents present at "back to school" night. Prep schools are so named because they "prep for the SAT" from 8th grade on.
If by "prep school", you mean the elite, independent, private schools like Andover, Groton, St. Paul's, and Lawrenceville they do a lot more than simply prep for the SAT. And they were called prep schools long before the SAT existed.
Getting back to the HS at which my wife teaches, I'd venture to guess that 90% of the families could afford to send their kids to an elite private school, but they, following the excellent example of BobPSU92, choose not to.