CLINTON, N.C. (CBS Local) — Three students in North Carolina were hospitalized Monday after part of the roof of the school’s gym was blown off by strong winds.

Surveillance video shows 21 students running for their lives as debris started flying into the gymnasium at Union Intermediate School in Clinton as the roof came down, leaving a large hole in the floor.

Officials say a microburst, described as a localized column of sinking air within a thunderstorm, slammed into the school at about 2 p.m.

“It was just bam! We had no warning,” Principal Dondi Hobbs told CBS affiliate WNCN. “Nobody received any alerts on phones or anything before we were in action mode.”

Three students were sent to the hospital before being cleared by medical staff.

There were more than 450 students in the third, fourth and fifth grades inside the school at the time.

“I talked to a third grade teacher this morning who was here. She says her students behaved so well. They took it all in stride. They were so mature about it,” Wendy Cabral, a spokeswoman with Sampson County Schools, told WTVD.

Officials say the gym is still structurally sound despite the extensive damage.