The call came in shortly before noon. Parents were notified via text message of the lockdown situation.
"He had to talk him down and he was able to get the weapon away from the suspect," said Police Sgt. Sean Russo.
Kleiner says Mitchell pinned the man down until police arrived.
The suspect was taken into custody and charges are pending. Police say he has ties to the school.
Students and teachers were locked down in the school while authorities searched for other suspects. None were found.
"My belief is that the suspect went right to the administration office, and the children were never in danger," said Orangetown Police Sgt. Sean Russo. It appears no shots were fired.
A district spokeswoman could not confirm how the gunman entered the building.
Parents say the door is usually unlocked and all visitors are to sign in before entering the building.
The school was locked down until 2:20 p.m. Parents had to sign out their children before they could go home.
Parents are hailing Mitchell a hero.
(The AP contributed to this article.)
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