Two years ago, when Mayor de Blasio and Schools Chancellor Carmen Fariña lifted the ban on cellphones in New York City public schools, they also instituted what they called a “Misuse It, You Lose It” policy to prevent cyberbullying — which has increased 351 percent in that time.

According to a Post analysis, there were 804 reported incidents in the 2015-16 school year, compared to 686 the prior year and 178 in 2013-14, the year before the cellphone ban was lifted. The most common types of incidents involve fat-shaming and harassment over race, gender and sexual orientation.

Some experts believe the spike in numbers is due to increased vigilance: We are more aware of cyberbullying, and so more likely to report it. But this is probably wishful thinking.

The case of Megan Meier, the teen who committed suicide after classmates bombarded her with hurtful messages on MySpace, happened in 2006. It was in 2010 that Tyler Clementi jumped off the George Washington Bridge after his roommate used a computer to spy on him and then shared footage with other classmates at Rutgers University.

Heck, there was a 2011 movie called “Cyberbully.” This isn’t a new phenomenon.

The more likely scenario is that cyberbullying in New York City schools has risen in tandem with the number of hours a day that students have access to cyberspace. In other words, thanks to the fact that cellphones are now allowed in school, students have another seven or eight hours each day in which to insult one another online.

A 2011 study in the journal Children and Society found that “frequent Internet users are more likely to perpetrate acts of cyberbullying.” A 2011 survey of 500 American teens sponsored by the Telecom firm Openet found that 42 percent of heavy cellphone users — those who send more than 60 texts on a typical school day — have engaged in negative or inappropriate activity on their phones compared to just 18 percent of light users.

But heavy cellphone use is not only correlated with bullying. It’s correlated with being bullied. According to the survey, “46 percent of heavy users experience cyberbullying on their cell phones, a much higher rate compared to just 23 percent of teenagers who are within the ‘normal usage’ bracket.”

Would these kids simply shift their cyberbullying to another time of day? Doubtful: There are too many hours to make up for.

Moreover, studies show that adult supervision of technology makes cyberbullying less likely. So it’s harder to do on home computers or when parents are looking over their kids’ shoulders.

While there are supposed to be strict rules about the use of phones inside of city schools — depending on the school, they may be restricted to lunch or free time — students will tell you that plainly isn’t working. In any classroom where cellphones are allowed — from elementary school through college — students will be busy tapping away when they’re supposed to be learning.

And the more opportunity they have to tap, the more likely they are to be tapping things they shouldn’t be.

In fact, schools elsewhere have instituted bans on cellphones as a way of reducing bullying (both real life and in cyberspace). A high school in Alabama launched a ban in 2014 after students kept going on the site Yik Yak to post abusive comments about other students and even teachers. At one school in New Zealand, the principal explained, “Cellphones were sometimes the source of the problem.” In addition to mocking others online, students were using phones to film fights.

The use of phones to record other students is perhaps one of the biggest ways these devices can become problematic in schools. In a place where students are supposed to feel safe physically and emotionally, more and more students know that any comment in a classroom or social misstep in the cafeteria can be recorded and posted on Instagram before the end of the period.

This kind of environment isn’t conducive to learning. There’s plenty of evidence that cellphones are detrimental to academic achievement, particularly for students who are less advantaged to begin with. But it’s clear that allowing cellphones in schools has made what is an already difficult environment much worse.

Naomi Schaefer Riley is a senior fellow at the Independent Women’s Forum.