JACKSONVILLE, Fla. – Firefighters responding to a mass casualty event at Highlands Middle School on Friday afternoon took 39 seventh-grade students suffering effects from pepper spray to area hospitals, according to Duval County Public Schools.

The incident happened in the gym during a fifth-period physical education class. Teachers and staff immediately evacuated from the gym to the football field.

“I was coughing a lot ... it was very hard to breathe,” one student who was treated at the hospital and quickly released told News4Jax.

Sky 4 aerials over the school about 2:15 p.m. showed dozens of students in bleachers on the athletic field and the several rescue vehicles, fire engines and police vehicles at the school. Within 30 minutes, it appeared students were being led back into the building.

“We are in the process of reaching out personally to all families of students who appear to be suffering from any of the effects of the spray,” parents were told in a robocall. “We will inform you if your student is safely in class or if he or she is getting medical attention.”

Families of seventh-grade students at Highlands who have not heard from the school are encouraged to call the school at 904-696-8771.

The source of the pepper spray was still under investigation, although a student told News4Jax that it was sprayed from the bottom of the bleachers in the gym.

Parents said the school didn’t notify them of what happened. One parent saw his daughter being taken out in an ambulance and no one could tell him where she was being taken because the school didn’t know.

School Superintendent Diana Greene is expected to address the event at 4:30 p.m. at UF Health Jacksonville. News4Jax will carry her statement on air and online.