Is High School a Privilege?

The other day I was pondering why private school costs about half as much as public but on whole consistently produce better results?  Sure I've noticed there are more secretaries, principals, support staff and assistant teachers in public schools but the bureaucracy costs associated with a bloated educational system is only part of the problem. It's more likely the cost associated with unfunded Federal mandates that private schools are exempt from, are at the root of the problem.

If a school doesn't accept federal funding it doesn't have to comply with IDEA (Individuals with Disabilities Education Act).  This alone creates massive savings.  However, I think it's safe to say that private schools do better because they don't have to accept every student.

Here is my thought.  Should moving up to high school be automatic?  Should pretty much all 8th graders go on to high school?  Should the government be required to provide a high school education to age 21?

For example does it make sense for kids with fetal alcohol syndrome to be sitting next to a college bound senior?

I don't have a solution, I just know that an oppositional defiant disorder student "504 - Accommodation" that comes late to school virtually everyday, talks on his cell phone during class, has been arrested several times and is pretty much the worst possible kid to have in class is required by law to be educated is a political correctness nightmare.

The conundrum... It's cheaper to educate then incarcerate and clearly education is much better for society.  Education for most is key but public education suffers when good intentions result in unintended consequences.  What can be done?  All I know is change is needed.  What are other countries doing?