It happened before 11 a.m. Wednesday at the Whitney M. Young Jr. Community School (P.S. 15) on Stegman Street.
The school was evacuated after a release of pepper spray in the building.
As police and fire officials investigated, a number of students were evaluated at the scene.
Nineteen were taken to Jersey City Medical Center as a precaution. All were treated and released.
The all-clear was given at the school, and classes were expected to resume early Wednesday afternoon.
The incident remains under investigation.
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