Parents at the posh Columbia Grammar & Preparatory School are outraged they were never told of a fourth "R" being added to the curriculum: raunch.

In addition to the usual reading, ‘riting and ‘rithmetic, the school this month launched lessons on porn — without informing families or allowing them to opt out, parents fumed.

When juniors at the $47,000-a-year Manhattan school showed up for a health and sexuality workshop, most thought it was "just going to be about condoms or birth control," a student told The Post.

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Instead, it was something called "Pornography Literacy: An intersectional focus on mainstream porn," taught by Justine Ang Fonte, who’s the director of Health & Wellness at another elite prep school, Dalton.

The often-explicit slide presentation and lecture by Fonte to the 120 boys and girls included lessons on how porn takes care of "three big male vulnerabilities"; statistics on the "orgasm gap" showing straight women have far fewer orgasms with their partners than gay men or women; and photos of partially-nude women, some in bondage, to analyze "what is porn and what is art."

Fonte’s presentation, some of which was seen by The Post, included a list of the most searched pornographic terms of 2019, including "creampie," "anal," "gangbang," "stepmom" and more.

One slide cited various porn genres such as "incest-themed," consensual or "vanilla," "barely legal," and "kink and BDSM" (which included "waterboard electro" torture porn as an example).

"We were all like, ‘What?'" a female student said. "Everyone was texting each other, ‘What the hell is this? It’s so stupid.’ Everyone knows about porn. The worst part of it was that it took place not long before the AP tests and I had to miss both my AP classes for this."

One part of the porn presentation involved something called the "marketability of Only Fans," the hot new app used mostly for sex work. One slide included a photo of a pretty young woman who appeared to be promoting OnlyFans-type work.

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"I identify as non-binary," she is quoted as saying, "but because that hasn’t hit the general consciousness of the adult industry, I say ‘girl,’ because that’s what people who want to buy my content will be looking for."

The female Columbia Prep student said most of the kids, aged 16 and 17, watched the lesson on Zoom from home — which is what alerted some parents to it — but some were at the school and made to assemble in the gym together to watch it on their laptops.

"We were all so shocked and mortified," the girl told The Post. "We were all like, ‘Why are they doing this? Why do they think it’s OK?’

"We were supposed to answer questions about the porn stuff in the Zoom chat but we were all side-chatting in group chats and tons of kids thought it was so dumb that they sent the link to their friends all over the city and they were all logging on with the password."

The girl spoke to The Post with her mother. Both spoke on the condition of anonymity.

"No one wants to be cancelled or lose their livelihood and that can be done in an instant," the mother said. "Most parents feel the same way I do about not going public but at the same time we’re incredibly frustrated by what’s going on. None of the parents knew this was planned. We were completely left in the dark. It makes us wonder what else the school is up to."

Another parent of a middle-schooler at the pre-K-12th-grade school said, "It’s outrageous that the school is introducing pornography into a mainstream classroom and starting to indoctrinate kids. The goal of this is to disrupt families."

"Why is the school making porn a priority as opposed to physics, art, literature or poetry?" she asked.

Three other parents who spoke to The Post said they asked school administrators to show them content from the presentation after it took place but were rebuffed. One mother sent a letter to the school and was granted a discussion with administrators by Zoom, she said.

READ THE FULL STORY AT NYPOST.COM