City school students had their spring vacations extended by a day after vandals deflated the tires on all 88 school buses in the fleet.

The district canceled classes Monday after bus drivers reporting for work found the damage.

"We're treating this situation very seriously," said Bay City Public Schools Superintendent Carolyn Wierda. "We will proceed in any way possible to take action against whoever is responsible."

The vandals hit sometime over the weekend, since employees had Good Friday off and everything seemed normal the day before, said district transportation director Michael Gwizdala.

He said there was no evidence that the fence around the bus yard was damaged, and the vandals left behind nothing obvious but the flats.

"They opened the valves and took the core out," Gwizdala told The Bay City Times. "There were no punctures or anything."

Similar bus attacks happened around southeastern Michigan in December and January.

School officials in Bay City expected buses to be back on the road Tuesday.