Taken to extremes, liberalism eats its own.

Exhibit A is the decline of one of the most prestigious and proudly liberal private schools in the city.

Ethical Culture Fieldston in Riverdale, founded in 1878 by the son of a rabbi as a model for social justice, racial equality and intellectual freedom, now is convulsed by allegations of anti-Semitism, racial discrimination lawsuits, quarrels over identity politics, a Diversity, Equity, and Inclusion Department run amok and a union­ized faculty impervious to criticism.

Disgruntled parents are considering ways of extracting their children, angry donors are closing their wallets and distraught teachers are resigning or taking leaves of absence.

For some parents, concerns were crystallized when Fieldston students became aware that a YouTube video, “Secrets of the Living Dolls,” featured a new member of the faculty, a self-described “large black man” with a fetish for dressing up as a “sexy white woman” in a lifelike latex mask and breasts.

“I’ve fought for LGBT issues,” says one Fieldston parent, “but this is indicative of the weird things happening around the school which are rooted around intersectionality and an obsession about identity politics and race.”

A school insider says the video “bothers a particular group of parents, but in the hierarchy of problems at the school, I don’t consider this to be a particularly big one.”

Far more troubling is an atmosphere at the school of “rampant, systemic anti-Semitism.”

The issue came to a head this past November when Fieldston invited Kayum Ahmed, who works for George Soros’ Open ­Society Foundations, to speak at an assembly, and he likened Israel to Nazi Germany.

“The victims of the Holocaust and violence have become perpetrators of violence against Palestinians,” Ahmed said, prompting the Anti-Defamation League to blast him for an “outrageous … virulent [attempt] to blame the victim.”

The school erupted. Parents noted that, while the school had no problem inviting the controversial Ahmed, when a female student wanted to invite Jonathan Haidt, author of “The Coddling of the American Mind,” “administrators threatened to boycott” because he was seen as conservative.

A request for an interview with Head of School Jessica Bagby was referred to an external communications agency Wednesday, but her office pointed to a letter she sent to parents a week after the assembly, decrying Ahmed’s remarks as “deeply hurtful.”

“We will not accept anti-Semitism. We will not accept racism … sexism … homophobia … transphobia … xenophobia. We will not accept hatred in all its ugly shapes and forms.”

In January, to placate angry parents, two rabbis were invited to speak at Fieldston.

But the event was marred by a protest staged by history and sex-ed teacher JB Brager, a self-described “trans-Jewish scholar” who goes by the pronoun “they.” Brager “flipped the bird” at the rabbis, according to two parents, who viewed a video taken by a student. A few days later, Brager left the school.

One of the most contentious issues at Fieldston has been the introduction of affinity groups, separating children by race.

The groups “are rooted in a ­sociological model of whiteness that completely disregards the lived Jewish experience and, for a community with an above-average population of second- and third- and even first-generation Jews, that was deeply problematic,” said one insider.

“There are relationships with the school that will never be repaired. It really represented a turning point in the trajectory of the school and uncovered a degree of anti-Semitism that Fieldston [previously] has been immune from.”

A mother says the groups were a “pretext … They take kids of color and make them angry … It is the start of the blame culture. White people are to blame. The poison is far-reaching.”

Fieldston is paralyzed by “wokeness,” the mother says.

Another member of the community blames “an extreme far-left group of teachers … and the most important identity at the school is being a person of color or gender-nonconforming … Boys and white people are demonized. If you’re a straight white Jewish boy, you’re the worst.”

A big problem at the school is the fact that the teachers are unionized, says organizational psychologist David Nygren, who conducted a review of Fieldston’s corporate governance in 2017.

“They are a unionized faculty and they might have their own set of values to champion, which may be at odds with the brand promise Ethical Culture delivers to students,” he says. In other words, firing or disciplining nonperforming teachers is difficult.

Another insider lays some blame on “an age when so many parents think their kids are flawless … But the toxicity that exists at the school makes it impossible for parents and teachers to have collaborative constructive relationships …”

The political angst at the school spilled over when Donald Trump was elected president. “The school was closed for days,” says one parent. “The first day, teachers didn’t come to class, they protested the second day and the third day there was counseling … What kind of fragile snowflakes are we cultivating that they can’t deal with a presidential election that doesn’t go their way?”

The irony is that the indoctrination appears to be backfiring, with some students quietly adopting conservative views.

“Moms were coming to the school crying that their kids were growing up so conservative because that was their rebellion against what was going on with the school,” says one insider.

The irony is delicious, but for many families, the decline of the school they love is devastating.

Past hits Bloomy

We shouldn’t be surprised by Mike Bloomberg’s lame performances in the Democratic presidential debates.

In 40 years of running his own business, the autocratic billionaire has surrounded himself with supplicants.

No one challenged him and that was the way he liked it.

Not for him the rigorous quarterly earnings reports that keep public companies accountable to stockholders. The only investor Bloomberg had to answer to was himself.

He is even sheltered on the home front.

Where other candidates live with spouses who are willing to deliver the sort of savage truths that prevent repeat errors, Bloomberg, 78, is not married.

His girlfriend, Diana Taylor, who describes her role in their relationship as that of a “unicorn” and step-“friend” to his adult children, is hardly going to rock the boat.

He’s spent his life avoiding conflict and now he can’t defend himself.

Flying CEO a hypocrite

Well, look here. Just last month BlackRock boss Larry Fink lectured the world’s most powerful CEOs on the threat of climate change: “Every government, company, and shareholder must confront climate change.”

Then he hopped into his greenhouse gas-spewing private jet and flew to Australia this week to proselytize sustainability, according to the Australian Financial Review’s Joe Aston.

Fink’s Gulfstream 650 also was spotted in Zurich during Davos last month and in Frankfurt in September.

This would be the same Larry Fink who let The Post know in 2011 that he flies only commercial. “Fink prefers coach,” headlined the report of BlackRock proudly eschewing private jets.

“He flies commercial, and yes, he takes off his shoes and stands in line with the rest of us,” a close colleague of Fink said at the time.

Witness the billionaire’s creed: Do as we preach, not as we do.