"All of a sudden you hear this big boom during 4th period," student Chris Coler told CBS Sacramento.
It wasn't construction, or a band practice gone wrong. It was a toilet, laced possibly with fireworks, that blew to smithereens.
The culprits? Two Nevada Union High School students, who now face felony charges of possession of an explosive device on school property, criminal conspiracy, felony vandalism and probation violation.
"It was powerful to completely blow up a toilet and shatter the lighting fixture above the toilet," Nevada County Sheriff's Dept. spokesperson Alicia Milhouse told the station.
"The blast was heard throughout campus," Sheriff's Sgt. Bill Smethers told The Union. "They are lucky they didn't seriously injure somebody."
The sergeant told the paper that the explosives, believed to have been a M-80 firework, blasted the toilet seat into the ceiling light fixtures.
One 16-year-old juvenile has been booked into Nevada County Juvenile Hall, and his alleged 15-year-old accomplice has been released in his parent's custody.
In June, 18-year-old Tyell Morton faced eight years behind bars for placing a blow-up doll in the girls' bathroom as a senior prank.
"It's not right. It was a senior prank," Morton told WTHR. "They're blowing it out of proportion. I didn't hurt anybody, I didn't intend to embarrass anybody. What did I do wrong, you know?"
CORRECTION: A previous version of this story claimed Nevada Union High School was in Nevada when it is in California. We regret the mistake.