Which viewpoints should be included or excluded in education?

"'Not everyone believes the Holocaust happened...And you have your thoughts, but we are a public school and not all of our parents have the same beliefs.'

[The principal] went on to say that as an educator he had 'the role to be politically neutral but support all groups in the school.'”

There is an obvious fallacy here, but also an issue that can be genuinely difficult when applied to other cases that are more disputable than the existence of the Holocaust. First the fallacy. Consider this:

"Not everyone believes the Earth is round."

That does not mean the views of the flat earthers should be treated as serious alternatives to the conventional wisdom in geography and science courses. Indeed, teachers should feel no obligation to cover them at all. The fact that some people believe X does not mean that it is even minimally plausible, or deserves coverage by teachers.

But things get harder when we move on to other cases. Consider this one:

"Not everyone believes that the Great Depression was caused by insufficient government regulation of markets, and ended by the New Deal."

These beliefs are probably held by a large majority of Americans who have views on the issue at all. But they (particularly the second) are also rejected by numerous leading economists and economic historians. Thus, it would be irresponsible for courses on history and economics to present the conventional wisdom above as an indisputable truth (even though a good many do just that). The fact that most people believe X and many educators treat it as an obvious truth doesn't mean they are right to do so.

And this one:
"Not everyone believes that Brown v. Board of Education was correctly decided."

Brown is the most revered case in the constitutional law canon. Most of those who reject it (both now and historically) are segregationist bigots. However, Brown was also opposed by such people as the great liberal Harvard law professor Herbert Wechsler (who opposed segregation, but also wrote a famous article arguing that Brown was wrong as a matter of legal reasoning) and the radical African-American law professor Derrick Bell (probably the most famous black legal scholar of the 20th century). Bell argued that the Court should have stuck to "separate but equal," but enforced the "equal" part much more aggressively, thereby (in Bell's view) resulting in better education for black students than actually occurred in the aftermath of Brown.

Should constitutional law courses cover Wechsler and/or Bell's perspectives on Brown? A good many actually do. It is, in fact, common for constitutional law textbooks to include material on them. I believe Bell and Wechsler are at least worthy of consideration in a way that Holocaust deniers are not. But there's also a nontrivial argument that we shouldn't give these ideas legitimacy (and thereby risking indirectly giving ideological legitimacy to racist segregationists).

The broader point here is that one of the most important choices educators make is deciding which viewpoints are legitimate enough to include in a course, and which are so ridiculous and "beyond the pale" that they should be barred. Disagreement over which is which is at the heart of many debates about education policy and possible ideological bias in academia. You can't cover every conceivable view, so exclusion of some alternatives is unavoidable. But our decisions on that are inevitably influenced by our own viewpoints, and hard to keep free of bias.