I don't see how this should require any country to allow dangerous children into schools. Suppose some parents insist on sending their kid to school with a gun

I don't see how this should require any country to allow dangerous children into schools. Suppose some parents insist on sending their kid to school with a gun. Should the government be required to accommodate this kid? Obviously not: the government is perfectly within its rights to set limits on things, and not allow kids to attend who are a clear and present danger to the other children. Kids carrying guns are obviously a danger, so they're not allowed. Kids who are unvaccinated are also a danger, so they shouldn't be allowed, though with a high enough vaccination rate we can make an exception for someone with a valid medical reason. The government isn't preventing these kids from attending school; the parents are.