A 15-year-old boy was stabbed by another student at their Brooklyn public high school Tuesday in a burst of gang-related violence that prompted a nearly daylong lockdown of the building and left parents fuming.

Anxious parents flocked to Edward R. Murrow High School when news of the disturbing violence broke — while the suspect, also 15, fled the building and stayed in the wind for hours before turning himself in at a police precinct stationhouse Tuesday evening.

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., police and students said.

The victim was rushed to Maimonides Medical Center’s children’s hospital, and was expected to recover, cops said.

Meanwhile, a mass of cops descended on the Midwood school — which counts more than 3,500 students, and does not have metal detectors installed, according to sources.

Principal Allen Barge said in a letter to parents 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, adding that lessons would resume after.

“This is ridiculous! We’re paying taxes for this?! What the hell is this?” one frustrated dad, Patrick Caesar, 52, said as he stood outside the building in the early afternoon, wanting to pick up his 17-year-old son.

“Why are they checking them now for weapons? Why aren’t they check them before one guy stabbed the other kid?” he fumed. “It feels like nobody’s in charge here!”

Yolanda Washington, 38, the mother of a 14-year-old freshman, shared similar sentiments as she stood outside shivering, holding the hand of her 5-year-old daughter.

“Four-thousand students?” she seethed. “We’re out here waiting for them to search 4,000 students? I’m concerned!”

The two parents were among the throng of relatives of students who gathered around the school for hours – some standing out in the cold, and others in their cars with their hazard lights on, completely blocking off East 17th Street, near Avenue L.

“It’s on hold. The whole school has been on hold for a while,” said Melyna Ponce, 16, a junior who arrived late Tuesday and wasn’t allowed to get inside after cops blocked the entrances.

Ponce was texting with her friend, also a junior, who was inside the school – and recounted her pal’s experience.

“They’re not letting us out of our classrooms,” she said. “They’re not letting us use the bathroom. They keep going back and forth.”

Sophomore Owen Matthews, 15, said even the presence of metal detectors — which would have caught the knife, according to sources — would do little to keep weapons out of the hands of students.

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

“We’re definitely going to have the metal detectors tomorrow!” the teen added. “They only have them after stabbings like this.”

In an early afternoon statement, city Department of Education spokeswoman Jenna Lyle said that “the safety and wellbeing of our students is our absolute top priority.”

“Following an incident in the school, NYPD School Safety immediately responded, and NYPD and EMS are on site. The building was placed on a brief lockdown which has now been lifted.”

But about a half-hour later, the lockdown appeared to be ongoing, as relatives continued waiting outside the school.

Ivan Osorio, 26 – the brother of a 13-year-old freshman – said he showed up to pick up his sister, who initially told him students were being dismissed.

“She said, ‘My school is in lockdown,’” Osorio said. “I jumped on a bus as fast as I could to get over there. They told me they were letting some kids leave, but then they stopped that – the community affairs cops told me I had to wait.

“I used to go to school here,” he added. “Nothing like this ever happened here, so we’re all terrified.”

By dismissal time, parents could be seen yelling at cops – demanding access to their children or to speak to the principal – before a school safety commander assured them the kids would be dismissed in five minutes.

Sure enough, a rowdy crowd of kids left the school from the Chestnut Street side of the building around 3:30 p.m. – some cheering and jumping up and down.

A group of teen boys reported being held in a hot room for hours, with minimal bathroom breaks – and that cops physically searched them individually.

Police confirmed the suspected stabber had been taken into custody, but did not announce charges, or release his identity, due to his age.

Sources said the Brooklyn institution – which opened in 1974 and is best known for its music, theater and art departments – was also short on school safety agents, with only seven on duty Tuesday, four short of the required 11.

The city currently has 3,800 school safety agents staffing all of public schools, with between 5 and 7% set to retire at the end of the year, a law enforcement source said. Historically, 5,250 agents have been in place citywide – a number that began to erode during former Mayor Bill de Blasio’s administration, according to the source.

That shortage comes as the newest class of 250 school safety agents was recently axed by City Hall, with Mayor Adams saying last month parents may have to step up to make up for the shortage.

“They’re taking away school safety agents and they want us to come down here and be school safety agents?” asked Caesar, the irate dad, who works as a HVAC technician for the Local 20 union.

“I would gladly come down here and be a school safety officer, but they wouldn’t want to pay me the same salary I make,” he quipped.