Students reported hearing a very loud popping sound at about 8:30, said Ketti Davis, superintendent of Central Unified.
Davis said students and staff acted fast to lock down their classrooms and send messages out. She said a short time later, school officials learned the noise was firecrackers being set off in the restroom.
Jiwan Kaur described the terrifying moments she learned her son's school was placed on lockdown: "I was shaking. My legs were shaking. I was scared."
Though he later said he was safe, she rushed to pick him up.
"I don't want to leave my kid on the campus, you never know what's going to happen next," she said.
Fresno police are working with the district, going through surveillance video to track down those responsible.
"It really scared a lot of parents, a lot of administrators and put us on high alert. I had a parent personally call me in tears, thinking there was a shooting at the high school," said police chief Paco Balderrama.
Balderrama said this isn't the first incident of this nature his detectives have investigated since the tragedy in Texas.
While the severity of charges those responsible will face is unclear, he says the mere act of setting off firecrackers in a building is against the law.
"It was done in malice, it was done to scare the school. It did scare the school, it caused it to shut down. A lot of unnecessary resources were expended on this incident and nobody was in danger. It was just people playing a prank and pranks have consequences," said Balderrama.
The prank came at a time when students and staff members are already on high alert following the deadly mass shooting at an elementary school in Uvalde, Texas.
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The district said mental health professionals and counselors were available to students in need of services.
Central East High wasn't the only school dealing with a lockdown situation on Friday.
The Hanford Elementary School District went on a soft lockdown after a student reported concerning statements to Hanford police.
They say they were able to determine there was no active threat and the comments that were made were from a teen in Coalinga.
Law enforcement agencies across the Valley are reminding parents to talk to their kids about the consequences of actions like this.