A subbasement at Libertyville High School flooded early Thursday, prompting officials to cancel classes and on-campus activities for the day.
Maintenance workers discovered rising water in storage space beneath the arts wing about 6:30 a.m., Libertyville-Vernon Hills Area High School District 128 spokeswoman Mary Todoric said.
With students and staffers set to arrive shortly, administrators opted to shut down the building.
"We didn't have time to assess that it was safe before kids got here," Todoric said.
Police officers, firefighters and engineers responded quickly. Contrary to media reports, the building on Route 176 did not sink, collapse or catch fire.
Assistant Fire Chief Ken Komers suspected the water contained sewage because the village's sanitary sewers were backed up Thursday morning.
Several feet of water had risen in the subbasement within a few hours of the discovery. The space that's under water contains reams of paper, school supplies, maintenance equipment and other gear.
Behind the school, Butler Lake overflowed its banks and was covering a large portion of the north parking lot.
"I've never seen it come out this far into the parking lot in the years I've been here," Todoric said.
School officials will meet throughout the day to determine if classes will be held Friday.