A Long Island school board last night authorized an independent investigation into the explosive claims that its legendary football-coach-turned- schools-superintendent pressured subordinates to inflate a star player’s grades so he could get a Syracuse University scholarship.

The Bellport board hired attorney Bronwyn Black to lead the probe of Joe Cipp Jr. and others who are accused of having lineman Ryan Sloan’s grades inflated.

More than 300 people, some wearing Bellport football jerseys and jackets, turned out for the unanimous vote.

“I categorically deny these allegations. I never instructed or authorized anyone to change the grades for him or any other student,” Cipp told the board.

But a woman named Miriam Rivera, who identified herself as Sloan’s ex-girlfriend, said, “I’m going to say what [Sloan] told me. He told me that his grades were changed. He told me that himself.”

Her claim was met with derision from Cipp’s backers in the audience, many of whom screamed out, “Bulls–t!’’

The Post on Monday revealed the grade-fixing allegations in a story citing ex-Bellport Principal Kevin O’Connell’s intent to sue the school district over his firing.

He claims he was canned for not playing ball with the school’s grade-changing culture.

O’Connell said that Sloan’s grades later were changed by others — with one F becoming a D and two D’s being turned into C’s.

Ex-football player Kevin Wilson last night credited Cipp with having helped troubled kids from Bellport’s rough northern area.

“Our superintendent put them in a positive direction, not to jail, but to college,” Wilson said.

“Not by fixing grades but through hard work and dedication.”

But Bellport resident Rocco DeVito said, “Staff and some townspeople are afraid to speak at these meetings for fear of reprisals and expectations of harassment in the workplace.

“They fear for their jobs if they voice a differing opinion.”