The problem with any government monopoly service is that once it realizes that it commands the violent power to force its customers to obey, the government service providers forget that they are supposed to be acting like a servant—serving paying customers.
Inevitably, power corrupts, and government servants transform into petty tyrants. They, after all, can exploit "legitimized" police power to fund their service and enforce their edicts. Taxpayers must donate regardless. Parents must submit their children nonetheless. Children must suffer the tyrants' abuse.
The free market has a simple remedy for an abusive service provider. The parent picks up her child, cancels the service, and takes her money and child to patronize a different provider. The donor withdraws his contributions and reinvests them in a more service-oriented charity. And if a better service doesn't yet exist, they invest in teachers to create a new one that matches their values and goals.