My plan? Parents should get a "voucher" for the full amount that the government would be spending on their children if they were actually in school, based on a simple formula. The amount of all school funding (in their locality) divided by all of the children eligible for school. Home schoolers get the same amount as any other schooler.
Of course, this is not going to reduce the size of school budgets any time soon, but this is not an overnight solution. I can envision a future where not a single student attends government school in county after county in the USA, while local school boards, States, and the feds continue to fund idle staff to babysit the empty buildings. At that point, it might be obvious to the overtaxed masses that this is an expense they can do without.
An earlier thought was dividing the school budget by the average number of students in attendance in government schools. And I still like that one.
Here is another Milton Friedman video from his PBS special Free to Choose: