Put 30 assorted kids in a class room, some with good behavior, others not so, and others even worse. Given the fact that the teacher spends part of the class time orginizing and "controlling" the class. Then figure in that for a 60 minute class, the teacher spends 5-10 discussing non related stuff. Add 5-10 minutes to transition between each class. That could eliminate 2 hours of "education" time every day. Add to that 1/2 hour for lunch, you've lost 2-1/2 hours. If you have a 7 hour day, you've lost 35% of your time.
Back to individual classes. If a teacher has 45 minutes in each hour of actual teaching time, they probably spend 20-40 minutes presenting the days material. Lets average that to 30 minutes. That leaves 15 minutes in every class for the teacher to spend with 30 kids individually. Now obviously some will grasp things right away and others won't. What happens to those that don't?
Now lets add something else to the equation. These kids for the most part are strangers to the teacher. No matter how altruistic a teacher might be, when all is said and done, a parent has more concern for the well being of their own child, than a teacher does.
So lets compare the homeschool classroom verses the school classroom. A homeschooling parent has 1-(15?) kids in class at any one time. Versus a school teacher who might have 30. A homeschooling parent of multiple kids might actually have assistance from one of the other kids, where a sibling might actually "tutor" another sibling. A homeschooling parent moves the child along at the childs speed, making sure the child understands and comprehends the course material. The parent can spend more time or less time on a particular area, depending on the needs of the child, versus the clock on the wall or the calendar. Maybe a child needs 65 minutes of intensive instruction 1 day on math, but only 20 minutes on history.
Most homeschooling parents study the material themselves, to make sure they can teach it to their own kids. Add the parental love factor, and I'd put most parents teaching their own kids, up against a schoolteacher any day.
Granted there are "some" parents who don't belong teaching anything. But there are also teachers who don't belong teaching either. Overall, I think most parents are capable of teaching their own kids thru HS. But don't confuse capability with responsibility. Without responsibility, the most educated parent is nowhere near as capable, as a parent with limited education but vast responsibility.
What must be kept in mind is that a school is a government agency.