As an example, we all benefit hugely from the existence of an EMS (911, ambulances, emergency rooms) system, even if most of us don't use it much. That's a positive externality that's worth the government making sure happens, as it wouldn't necessarily happen without government involvement.

Similarly, public schooling (or publicly paid school) provides externalities: a literate society, a basic level of civic and cultural knowledge that people can assume as a baseline, opportunities for social mobility. None of those exist with a voluntary system.

This proposal doesn't seem to pass that test: at least, nobody has proposed positive externalities that come out of this.