A 15-year-old at Brooklyn’s Erasmus Hall High School was in critical condition after a fellow student stabbed him repeatedly in the head with a pair of scissors. Click below to read the rest of the story.

Students said it started with a penny ante basketball beef: Chevoy Nelson, 16, was mad because victim Alfredo Allen stole the ball from him during a lunchtime pickup game at the Prospect Park South school.

Nelson – described by a school source as a special ed student who is emotionally disturbed – was arrested for assault with a weapon, the Education Department confirmed.

The two teens were playing basketball in the third floor gym during lunch when Allen took the ball and wouldn’t give it to the increasingly angry Nelson.

The furious teen punched Allen in the face then ran out to look for a weapon, witnesses said.

“He goes running around, asking everyone for a weapon,” said a police source. “He goes into a classroom and asks a teacher for acid. She obviously says no, but then she get distracted he grabs a pair of scissors and runs back to the gym. He chases (Allen) around, stabbing him.”

Wayne Morgan, an 18-year-old senior, said, “He stabbed the guy in the head like seven times. The teacher was there, but it’s not like he let it happen – the whole thing happened in like 5 seconds.”

Horrified students screamed and ran as blood spilled across the gym floor.

“I saw his head wide open,” said freshman David Scott, 15, who had been playing soccer.

“I was like, ‘oh my god!’ I ran out, I didn’t need to see that,” Scott said.

Freshman Jessica Sanchez, 15, was playing double dutch in the gym when she saw the attack.

“He went behind (Allen) and grabbed him by his neck and stabbed him seven times,” Sanchez said. “(Allen) was screaming, he was saying, ‘oh s–t.’ He didn’t believe it either.”

She said she fled.

“I saw all the blood and I ran. I didn’t want him coming after me,” Sanchez said.

Police said after the attack Nelson went to a school office and sat quietly, his hands covered in blood.