(a). The biggest thing keeping the public schools alive and well-populated is the property tax. It is one thing to sacrifice so your kid can go to a private or parochial school, but it is an order of magnitude more difficult when you must do this IN ADDITION TO paying a pile of money in property (or other) taxes to fund the local school district.
(b). In a sane world, the State would attach a small pile of virtual money to each individual kid for each K-12 school year (say, $7,500), and allow the parents to "spend" that money wherever they like...public school, private school, charter school, parochial school...as long as it is spent in a way that furthers their education as it would have been in a State school. The teachers' unions, which own the Democrat party, fight this sort of initiative with lethal force.
(c). There are many exemplary public schools and public school districts where students learn about as much as they would in a middling private school. (I happen to live in such a district). This has little to do with the school or its employees, and everything to do with the parents and families who live there.
(d). Every successful private enterprise has the ability to "flex its budget." That is to say, if revenues are down significantly in any year, they can make changes to expenses, and especially headcount, to remain profitable in such lean years. Public sector organization lack that ability, because they cannot easily lay people off (the employees have a virtual guarantee of lifetime employment). School districts have the additional impediment of parents who make a huge stink when their neighborhood school building is faced with a shutdown; these campaigns make school board members very reluctant to do the right thing and close school buildings, even when it makes a great deal of economic sense.
(e). As a result, when public school enrollment declines, the budgets stay the same and the taxpayers never get to realize the savings that would rightly result from a decline in the number of students. Near to where I live, in the Pittsburgh school district, school enrollment has declined by almost half in recent decades, and yet the budget remains steady or increases slightly. And I don't need to document here that academic results are not improving. On the contrary...