The principal at a progressive Manhattan private school told parents in an email last week that the Trump presidency was more troubling than Vietnam, the assassination of Martin Luther King Jr., the September 11 attacks and Watergate, The Post has learned.

Steve Nelson’s scorching missive managed to roil several parents at the $46,000 per year Calhoun School – no small feat considering the Upper West Side bastion’s blaring liberal bent.

“It was inappropriate, it was offensive, it was condescending,” said one parent. “This is a liberal school. So I guess that’s the approach. But this was too much. To compare this to 9-11 – I think that’s just too much.”

Nelson emailed a series of anguished ruminations on Donald Trump’s ascent to Calhoun parents, simultaneously calling for non-partisanship while skewering the billionaire’s policies.

In a message sent last week, Nelson, who also teaches journalism at the pre-K through 12 campus, noted his intimate familiarity with several recent catastrophes – including 9-11.

“I watched soot-covered New Yorkers grimly trudging north on West End Avenue on September 11, 2001,” Nelson wrote. “I am more troubled now.”

Elsewhere in the lengthy missive, Nelson acknowledged the theoretical need to avoid political bias in a school environment. “One in my position must be scrupulous in avoiding partisanship,” he said.

But he later asserted in the same message that “there are matters that transcend political diversity.”

“The ways in which equity and equality are now threatened are deeply troubling, including the constitutionally suspect and arguably discriminatory efforts to restrict or prohibit immigration based on religion and/or ethnicity,” he wrote.

Another parent told The Post Wednesday that she wasn’t a Trump zealot but was still made uncomfortable by the outward display of political allegiance.

“I just think this went too far,” she said. “I understand that this is a progressive school, but at a certain point you have to have some restraint.”

Nelson, who has been at the school for two decades and plans to retire this year, staunchly stood by his actions Wednesday.

“I’ve been at the school for 19 years and there was not a message that got a more positive response than this one,” he said. “If this upset a few people I understand. But you can’t make everyone happy all the time.”

“I not only stand by the statement but I wouldn’t mind it being published in full,” he said.

“One aspect of a progressive school is that we’re open to dissent and dialogue about everything,” he said. “I would discuss this with the families if they chose to come in and chat with me.”

But a parent speculated that failing to denounce Trump with sufficient vigor would lead to immediate leper status in key Calhoun social circles.

“Are you kidding me?” she said. “Here? You wouldn’t be able to show your face.”

Students at the West End Ave. school – which counts actor Ben Stiller and comedian Jordan Peele as graduates – eagerly backed their principal.

“You do know we go to a progressive school?” one male student, 16, told a reporter Wednesday. “They want kids informed on issues like executives orders. The day after the election we had an emergency assembly to talk about our feelings and what happened. Like in history class we talk about how the world was in the 1930s then and compare it to Trump. Similar economics and racial issues as in the 1930s.”

“My history teacher always finds a way to connect everything to Trump,” said a 15-year-old female student. “He will talk about ancient leaders destroying an empire then mentions Trump.”

“There are discussions between students, I mean we go to a pretty progressive school,” said a male student. “We are all in an agreement we don’t like Trump.”

Founded in 1896, Calhoun has an enrollment of 730 kids and prides itself on tackling thorny issues like racism head on.

As part of a 2012 initiative called “Deconstructing Racism,” the school commissioned a documentary entitled “I’m Not Racist…Am I?”

A Calhoun parent said she was surprised by the objections to Nelson’s approach. “You know what you’re getting here,” she said. “And we’re proud of that.”