A threat from a bullied student armed with an AR-15 assault rifle that sent a Long Island high school into a lockdown turned out to be a total hoax, school officials said. 

Long Beach School District Superintendent Jennifer Gallagher said an early-morning 911 call came from California — and did not involve “an alumnus” from the school.

“What we know is this: the police received a threatening 911 call from a number in California, with the caller identifying himself by the same name as an alumnus of the high school,” Gallagher wrote in an email to parents.

“The police were able to determine this morning that the caller was not the alumnus of the high school, who has a fairly common name,” she said. “The police are continuing to try to determine the true identity of the caller from California.”

The call sent Long Beach and Nassau County cops swarming to the high school on Thursday morning, with district officials calling for a lockdown of the building.

Gallagher said Long Beach Middle School and Long Beach Elementary School were put on a “lockout” — a call to seal the buildings to keep everyone out, even as classes continue.

“Some parents called during the lockout today, and wanted to come pick up their children,” she said. “As a parent, I absolutely understand that instinct, but it is really critical in these situations that we follow safety procedures as recommended by our partners in law enforcement.”

Gallagher said students are often unaware that a lockout is in place at a school because normal activity continues inside.

Meanwhile, she said cops cleared the high school of any threats.

“The police were then able to assure the security of the high school, going room to room with the high school administration to determine that everyone inside the high school was safe,” she added.

According to law enforcement sources, the 911 call said a 15-year-old student who was “tired of being bullied” threatened that he was armed with an AR-15 semiautomatic rifle.