The young cub tried to enter through the front door, but wasn't able to see the glass in front of it.
NJ.com reports that that, after eventually breaking through the door, the animal entered a classroom on the school's first floor.
Lawrence Hajna from the Department of Enviromental Protection said: "Evidently, he thought he could get out the other side. All indications are that it was an accidental entry."
Superintendent of the elementary school Timothy Nicinski said: "We're just grateful no one was hurt."
The bear is now thought to be in a nearby forest, after being safely led out of the school by police.
> Tranquillised bear falls out of tree on college campus
> 'Falling Bear' dies after Colorado car crash