Once banned by most school districts, cellphones now are as much a part of the class day for many students as No. 2 lead pencils were for their parents.

Teachers put them to use as metronomes in music classes, for instance, and give quizzes through a popular app called Kahoot! Parents text their kids to ask when they’ll be home or remind them to walk the dog when they get there.

Administrators, though, have become alarmed by students' increasing use of smartphones to bully one another during school hours, to engage in escalating disputes, and even to arrange fights that are then filmed and posted to social media.

Cellphones played a critical role in the May 3 brawl at Cheltenham High School that produced seven injuries, four arrests, and days of heated news coverage. According to the prosecution at a Family Court hearing for three of the students taken into custody, the fight was planned and recorded on cellphones.

The incident dramatized a question confounding school officials virtually everywhere: when, if at all, to allow the use of mobile devices by students who can’t seem to live without them?

In Cheltenham, the teachers’ union is urging the district to impose a total ban on cellphones at the high school, where students currently may use them in certain areas outside of class, such as the cafeteria.

“We asked for a ban because cellphones don’t just present academic problems,” said Jon Manser, vice president of the Cheltenham Educators’ Association. “There’s obviously the filming of the fight, kids texting one another and letting them know a fight is going to happen somewhere, or kids getting upset with one another via text message or Snapchat. Those are things in the moment we don’t know about, because they aren’t talking to each other out loud.”

Manser conceded a ban would be difficult to enforce.

Superintendent Wagner Marseille declined to be interviewed, but issued a statement that cellphone usage is part of “a comprehensive evaluation of school climate and culture” since the fight.

Senior class president Paige Kytzidis estimated that in a typical school day, she sends as many as 50 texts and 15 emails -- about student government matters, to friends, to teachers about assignments. While emphasizing that "obviously, we have to have zero tolerance for bullying," she said, “I don’t think banning is the answer. There are so many benefits to having a phone."

Perhaps not of the academic variety, however. In 2015, researchers at the University of Texas and Louisiana State University studied four British schools and determined that bans on cellphones raised exam scores 6 percent on average. They also found the negative impact of the devices was greatest on low-performing and low-income kids.

In the Philadelphia district, policies are set by individual schools and can vary from collecting phones at the front door to allowing them for classroom instruction. In Northeast High School in 2015, under a strict ban, an angry father engaged in a fervid argument with the principal over the confiscation of his daughter’s iPhone 6 for two weeks. The dad contended he wouldn't be able to contact his child in an emergency. The YouTube video of the quarrel generated nearly a half-million views.

“Parents want their kids to have the phone ... even though teachers are saying they're too distracting,” said Liz Kolb, a clinical assistant professor of education at the University of Michigan who has written extensively on the issue.

While most schools outlawed the devices a decade ago, Kolb said, today roughly 70 percent either have policies permitting them in certain settings, or are considering loosening their rules.

New Jersey schools have generally moved toward allowing phones for classroom instruction. The Winslow Township district in Camden County, however, has gone in the opposite direction with a complete ban. First-time violators lose their phones for 30 days; the second time, 60 days; the third, until the end of the school year, with a four-day suspension for any student who refuses to hand over the device.

Likewise, the Upper Darby School District banned mobile devices four years ago. But teachers and staff found themselves spending an inordinate amount of time arguing with kids who used them anyway, said Daniel P. McGarry, assistant superintendent of curriculum and instruction.  “A cellphone is like a kid’s life now,” he said.

So the district flipped to what McGarry called “a pretty liberal approach” that allows students to bring them to school and even walk down the hallways wearing headphones, as long as the phones are not taken out during a test, or used to snap pictures in the locker room.

Those rules have worked well in the high school, he said, but there are serious issues in middle school with "constant bullying."

“They’re using them during the day, all day,” communicating by text or Snapchat, said McGarry. He is working with middle-school principals on tighter cellphone restrictions that would stop short of a complete ban.

“All we were doing was chasing kids up and down the hall all day,” said Mark DiRocco, executive director of the Pennsylvania Association of School Administrators and retired superintendent in the Lewisburg Area School District in central Pennsylvania, referring to the ban at the high school several years ago. “It was difficult. We decided let’s teach kids to be responsible with these things.”

That’s a view most parents seem to embrace. “Today the mind-set is if something were to be wrong, I want to know about it as soon as possible,” said Amy Mendelsohn, whose son is in 10th grade at Cheltenham High School. “It’s seen as a safety issue. If he’s staying after school, he can text me, ‘Can you pick me up later?’ ”

Especially after the Sandy Hook tragedy, “some parents — understandably, I think — want phones on their [child's] person," said Danielle Arnold-Schwartz, an eighth-grade English teacher at Lower Merion’s Welsh Valley Middle School, where students are expected to keep phones in lockers unless they’re asked to bring them to class.

Yet Schwartz thinks that the cellphones are increasing attention-deficit issues, even as she occasionally employs them for tasks such as student reminders on a project.

Kolb, the Michigan professor, said some teachers ask students to place their phones on the corners of their desks, either turned off or flipped over so they’re available but not a constant distraction. Other teachers, she noted, even use red, yellow, and green lights in class to indicate when phones are forbidden, might be needed, or should be ready to be deployed.

Melanie Metz, 17, a junior at Harriton High School in Lower Merion, said kids use their phones, well, practically all the time, even surreptitiously taking a peek under their desks in class.

"They're just a regular part of culture now," she said. "I just don’t think anyone would follow a ban. Some people are very attached to their phones."