We can agree upon an authority who decides questions of truth or untruth.
You mean like a court? They don't really determine truth, usually there is a burden of proof standard that needs to be met. And that burden of proof depends on what is being judged. What they determine is what is deemed sufficiently "likely" to meet the burden of proof standard that applies.
We can use agreed upon processes to discover truth.
You mean like logic? There is no need to agree.
But yes, at the end of the day, what is real and what is not can devolve into philosophy, epistemology, etc.
Not really. Philosophy can help you to question your understanding of what you consider to be true, but the concept of truth stands independent.
For example, philosophy might help you to question whether what you see is really a chair, but truth can be contingent. "If what I see exists, it appears to be a thing called a chair based on my sensory information".
Or... to put it more concretely:
Premise: A
Premise: A->B
Conclusion: B
True. The conclusion is contingent upon the premises, but the conclusion is still true.
It is sensible to sanction the children of anti-vaxxers in modern metropolitan US school districts, but likely it is not sensible to do so in developing nations.
Let's take a hypothetical. Let's say that you signed a contract with a school whereby you pay them tuition and they admit your child, and their admittance of your child did not require vaccination. Is it then sensible for them to deny access?
Let's take another hypothetical. Let's suppose that your government seizes tax dollars from you to provide education to all children. And let's suppose that it is not illegal to avoid vaccinating your child. Is it "sensible" for your government to deny your child access without refunding the seized tuition? If so, which vaccinations? I'm not vaccinated against smallpox, and until recently I was not vaccinated against Hep-A (I got it to travel to China). I never got vaccinated against chickenpox (because I contracted it).
What is "sensible" can depend on other things besides whether the belief is false. For example, it can depend on whether they have a contract, whether they have paid, which vaccine it is and how much of a risk it poses. A tetanus vaccine? A flu vaccine? A chickenpox vaccine?
D) Not enough information