Alan wrote a heartwarming, if not tear-jerker of an essay about living with Crohn’s disease for a college application essay. He was proud of his opus detailing the struggles with the often crippling autoimmune disease that makes leading a normal teenage life all but impossible.

Only problem is, Alan is not a high school senior, but a former tutor-turned-ghostwriter in his early 30s who was paid $150 to write about this real-life situation by the applicant’s father.

The New York dad, like a lot of Alan’s clients, found him through his ad on Craigslist, the increasingly murky marketplace for legit and not-so-legit tutors, homework helpers and ghostwriters in dubious guises.

When his Crohn’s-afflicted high school pupil was accepted to his college of choice, he sent Alan a warm thank-you note — presumably one he wrote himself.

“A big segment of my clients are Ivy League-bound kids who are chess champions and want to go to Princeton and Harvard,” says Alan, an Ivy Leaguer who asked not to be identified by his real name.

Alan — who charges anywhere from $100 to $600 for assignments and holds a graduate degree in biology from a top NYC university — abandoned years of traditional tutoring in favor of academic dishonesty and moral collusion because, simply, “this is a lot more lucrative.”

He’s seen brisk business since going rogue in 2007, especially in the past three years, which he attributes to “referrals, increased competition, a high-stakes testing culture and anonymity online.”

“It’s a really busy season,” he adds. “I get more work than I can handle sometimes.”

In a city where even kindergarten admission is cutthroat, competition to get into the best colleges has never been fiercer, and cheating to get a leg up never more sophisticated — leading to greater demand for tutors like Alan who cheat for hire, say experts.

“It’s something that’s clearly a problem in New York,” says Tim Urban, a 31-year-old Harvard grad and the co-founder of Launch Education Group, a premier NYC tutoring service.

“There is definitely a market for unethical tutoring. And if there is a market, there is a supplier.”

Unscrupulous tutors are just part of a pervasive cheating culture in NYC. In June, dozens of high-achieving Stuyvesant HS students were found cheating on Regents exams via text messages. And last year, authorities blew the lid off an organized SAT cheating ring involving 20 students at Great Neck North HS. Ringleader Samuel Eshaghoff, a 2010 graduate of the school, was caught charging thousands of dollars to take the SATs for multiple teens; he accepted a plea deal to avoid jail time.

And the dishonesty continues to spiral out of control at the university level. In a new tell-all book, “The Shadow Scholar,” author Dave Tomar reveals how he was a “ghostwriter” who composed papers for thousands of college students over a 10-year period, again with desperate parents wielding checkbooks.

“What became obvious to me was this endless supply of paying work,” says the 32-year-old, who gave up his fraudulent life in 2010 and is now making his living as a legitimate writer in Philadelphia.

“A lot of that pressure extends from parents. They were frequently the driving force behind this.”

Tomar recalls getting desperate, bizarre messages: “I’m really panicking here, I need this paper for tomorrow” — from parents.

Urban, whose tutoring company is on the up-and-up, also fingers parents as the culprit. He says he’s been approached by so many grade-obsessed adults, his company now pre-screens every new client to avoid such immoral mine fields: “We refuse to do anything that we don’t consider ethical,” he says, admitting it’s hard to stay above the fray. “This is a whole different situation than it was 15 years ago.”

Charles, who didn’t want his name used and is now a well-respected doctor in his 30s, supported himself through medical school by tutoring the Upper

East Side’s top 1 percent.

When a struggling Dalton School student he was tasked to bring up to speed began to improve, the youngster’s bear-clawed mother dug in: Thus began a six-year saga that eventually netted him $150,000 and a down payment on his NYC apartment, before the sordid relationship ended in 2006.

The mother — a college professor — demanded Charles “tutor” her 15-year-old sophomore son by completing every homework assignment and writing every paper and college essay. While Charles originally worked for a tutoring agency, the arrangement soon changed at the mother’s behest. “The mother approached me, propositioned me. She said it would be a good deal for both of us if we cut out the middleman.”

Once the boy was off to his out-of-state private university, he flunked out after less than one year without the coddling of a tutor.

“I feel like I shortchanged him — but if it weren’t me, the mom would have found someone else to do the same thing,” says Charles.

And when the student was enrolled at a less-competitive school back in New York, Charles was pulled back in at the mother’s urging: “I was back in the picture in the same way as before: coming over five or six days a week. They paid for my apartment,” he says.

NYC teachers say they’re aware of the problem — and that they don’t need cheating software to recognize when a James Patterson is trying to pass himself off as Shakespeare.

“As a teacher for six years, the work they brought in more and more felt grad-school-like,” says one teacher at a competitive private school in Riverdale. “There’s a huge discrepancy between what was produced in the classroom versus a big [take-home] term paper that’s way too sophisticated for what a high schooler could produce.

“We would have staff meetings to discuss tutors: How do we grade this essay, knowing a tutor is crafting it? It puts teachers in an awkward position, because you don’t want to accuse the kid. Teachers can’t keep up with all the ways kids are cheating these days.”

While the teacher admits that it’s hard to prove a tutor — and not a student — wrote an original paper, she says that the growing problem has changed her approach to grading, which now includes more in-class essays.

“That guarantees that they are using their voice,” she says.

According to NYC Department of Education spokeswoman Marge Feinberg, there were reports of 427 infractions for scholastic dishonesty during the 2011-12 school year, an uptick from the previous year, resulting in “disciplinary” action.

Alan admits that his handiwork has raised red flags in the past. He recalls a history paper he wrote for a 16-year-old that raised suspicions: “[The] student got accused, relented and rewrote the paper himself. I told him rewriting it makes him look guilty.” Alan says no disciplinary action was ultimately taken against the student.

Stephen Friedfeld, a former admissions officer at Cornell and Princeton, is well aware of cheating high schoolers competing for admission to the nation’s top colleges: “You look at the test scores, grades and profile — and if the writing is incongruent between the student’s standardized test scores, grades and the essay, you know something’s going on,” says Friedfeld, now the COO of AcceptU, an admissions counseling site.

While applicants sign a form verifying that their college application is their own work, Friedfeld admits that “admissions officers cannot prove that a student did — or did not — write an essay.”

“If a student is having a tutor do work for her, good luck in college,” he snickers. “It’s just not for the student’s benefit.”

That’s something type-A parents have a hard time understanding.

“We have some mothers who feel like they’ve never wanted anything so bad in their life than getting their kid into Harvard. Usually the families who are very stressed out are in high-powered circles, and it’s bragging rights, too,” says Urban.

“Most of these parents aren’t terrible people or super-unethical in general. But it’s a phenomenon where they’re getting way too wrapped up in one thing: the culmination of whether they did a good job raising their child for 18 years. It’s a black-and-white finish line for them.”