A frightening bomb threat at a Tennessee middle school sent students scrambling outside — and ended with 21 children being rushed to the hospital after nearly an hour in the heat.

Three students were initially listed in critical condition after suffering heat-related illnesses during the emergency evacuation at Carter Middle School in Knox County on Friday, Sept. 4.

The scare began shortly before 10 a.m. when authorities were alerted after a threatening message was discovered written on a bathroom wall, according to the Knox County Sheriff’s Office.

School officials quickly evacuated the building, moving students to the football field at around 10:25 a.m. as deputies searched the campus for any possible danger.

Sergeant Wallace and Deputy Bowers conducted what authorities described as a thorough sweep of the school while hundreds of students remained outside.

They were allowed back into the building at approximately 11:27 a.m., meaning they had spent just under an hour outdoors.

But by then, several students were showing signs of heat-related illness.

Emergency crews rushed to the school and began treating students both near the football field and inside the building.

Rural Metro Fire said 21 students were ultimately transported to Dolly Parton Children’s Hospital for heat exhaustion, with three initially considered to be in critical condition.

“Students were outside per the school’s protocols while KCSO cleared the building and ensured the safety and integrity of the premises,” Rural Metro Fire said.

Officials said some students were treated inside a field house near the football field, while most received medical attention after returning to the school.

Despite the frightening scene, Rural Metro Fire’s Jeff Bagwell said the evacuation procedures worked as intended.

“The school reacted as they should,” Bagwell told WVLT.

“The system has worked flawlessly, so the public should know the system works,” he added. “The students are safe, and at no time were they in any harm or jeopardy.”

Knox County Schools Superintendent Jon Rysewyk said school employees worked to give students access to shade and water while authorities investigated the threat.

“Any time we are faced with a threat of this kind, we expect our schools to act swiftly and in partnership with our law enforcement partners,” Rysewyk said. “That is what Carter Middle School did.”

He acknowledged, however, that the extreme heat made an already tense situation worse.

“Unfortunately, the heat exacerbated an already difficult situation,” he said.

Rysewyk said roughly 20 students reported heat-related symptoms and were evaluated by medical personnel. Students expressing concerns were taken to the hospital “out of an abundance of caution.”

Fortunately, all of the hospitalized students were discharged before the end of the day and returned to the care of their families.

“Situations like these are never easy,” Rysewyk said. “I want to commend our school staff and emergency responders for acting with urgency to keep students safe throughout an extremely challenging situation.”

The Knox County Sheriff’s Office is continuing to investigate who was responsible for the threatening message.

“The Knox County Sheriff’s Office takes every threat involving the safety of our schools seriously,” the agency said. “We appreciate the cooperation of Carter Middle School administrators, staff, students and our emergency response partners throughout this incident.”