A 12-year-old girl was slashed by a fellow student in their Bronx classroom Wednesday morning — just a day after a teenage boy was stabbed in the hallway of his Brooklyn high school, cops and sources said.

The suspected attacker, 14, was taken into custody after slashing the pre-teen student in the right leg at J.H.S. 123 James. M Kieran School — during a fight that may have stemmed from a social media spat — at around 9:40 a.m., police sources said.

“Someone go get help!” the kids’ desperate teacher yelled when the violence broke out in the sixth-grade classroom, said witness Allison Sanchez.

“He went to stop the fight and we all went outside into the hall. It was so fast,” the 11-year-old, joined by her mom, told The Post outside the Soundview school. “After, they took us to another classroom and we had to sit there and wait.”

The attack came after an eighth-grader who had gotten into a scuffle with another girl was brought into Sanchez’s magnet class to “sit here and calm down,” she recalled.

A few minutes later, another eight-grade girl came into the classroom “and sat down at that table and they just started talking,” Sanchez said. “Then they started fighting and they pushed the table around. One had a knife.”

EMS took the victim to Jacobi Medical Center, where she was listed in stable condition.

Investigators were looking into whether the violence was sparked by a dispute that began on social media, law enforcement sources said.

A knife was recovered after the attack, according to police, who added that the public school does not have a weapons scanning system in place. 

Multiple NYPD School Safety vans lined the sidewalk of the school — on Morrison Avenue near Bruckner Boulevard — by early afternoon, as about two dozen parents congregated around the front door to pick up their children. 

Sanchez’s mother, Nely Cerano, 33, said that “there is always a fight” at the junior high, also known as the Bronx Urban Community STEAM Magnet School, which counts around 530 students.

“After school, every day, there are fights here … especially with the eighth graders,” Cerano said.

“There are no school safety officers in the school. Not usually… There’s one or two, but they’re only at the front desk,”  she added.

One father – who demanded to see his daughter and said he was going inside – was detained briefly by the officers.

Moments later, cops announced they would be escorting parents into the school four at a time to collect their children.

Paula Marmol, 41, who was there to pick up her 13-year-old daughter, said she received an email that the school was placed “on hold” after a student was stabbed, and that all the kids were safe.

Still, she said she was afraid to continue living in the Bronx and sending her daughter to school there — saying she was considering moving back to Pennsylvania, or to Queens.

“I hear that the gangs are coming back and I’m scared for the kids,” Marmol said. “I’m scared for my daughter. We just moved back here to the Bronx from Pennsylvania. I think we made a mistake.”

Students were dismissed normally shortly before 2:30 p.m., as school safety agents could be seen breaking down magnetometers and X-ray machines that were set up to check kids for weapons in the wake of the slashing. 

“They set up metal detectors and we had to walk through them,” said a 12-year-old student who identified herself as Lisa S. and spoke to The Post alongside her mom at dismissal.

“You had to put your bag on the belt and let it go through the machine so they could see in it. Everybody had to go through it,” she said.

Police said the 14-year-old suspect, whose name wasn’t released because of her age, was charged with assault, acting in a manner injurious to a child and criminal possession of a weapon

The bloodshed took place just about 24 hours after a stabbing at Edward R. Murrow High School in Midwood, where a 15-year-old boy was attacked by a classmate his own age in what sources said was a gang-related incident, according to cops.

The two boys, who knew each other, were arguing in front of lockers in a hallway on the third floor when one of them knifed the other in the stomach at around 9:20 a.m. Tuesday, police and students said.

Anxious parents flocked to the public high school when news of the disturbing violence broke — while the suspect fled the building and stayed in the wind for hours before turning himself in at a police precinct station house.

Police on Wednesday confirmed that the arrested teen was charged with attempted murder, assault with the intent to cause physical injury and criminal possession of a weapon, authorities said. 

The 15-year-old suspect was arraigned in the youth part of Brooklyn Criminal Court Wednesday and is being prosecuted as an adult, since the top two charges are felonies, according to the district attorney’s office.

Prosecutors requested that he be held on $100,000 bail, but the judge released him to “intensive community monitoring” – a form of supervised release for juveniles, monitored by probation.

In the aftermath of that assault, Principal Allen Barge issued a letter to parents stating that the NYPD was conducting a “reverse scan” of the building, in which students who were already in the building – and required to remain in their classrooms – were being searched for weapons.

During those searches, more than a dozen knives, two stun guns and several canisters of pepper spray were recovered from students, sources told The Post.

Students at that school – which also did not have metal detectors installed at the time of the attack – were greeted by the screening devices and a heavy police presence as they showed up for classes Wednesday morning. 

“We’re definitely going to have the metal detectors tomorrow!” one sophomore, Owen Matthews, told The Post Tuesday. “They only have them after stabbings like this.”

But Matthews said he doubted students would be deterred from toting weapons – even with the presence of detectors — which would have caught the knife, according to sources. 

“Kids just hide their stuff in the park and then they come and get it after school,” he said. “Metal detectors don’t do anything.”