It hires and trains their teachers, it decides on their curriculum, their class size, their textbooks, and on every other aspect of their education. We have a situation where unaccountable bureaucrats decide on the lives and education of these children. And even worse, the government has all but eliminated its competition. While private schools must earn their financing by the voluntary consent of students' parents, public schools take their financing by forced taxation. Private schools have to squeeze educational value out of every dollar they earn.
Public schools have only to make sure they spend their budget.
Private schools must pay their teachers by merit, for results, so they can hire, maintain and provide the best. Public schools for decades have refused to pay by merit, so they can hire and keep whomever they want. If a private school fails at educating a student, his parents can transfer him elsewhere. If a public school fails, well, that's just too bad. If a private school fails at educating its students, it soon goes out of business.