SCOTTSDALE, Ariz. -- Crews worked through the weekend at a Scottsdale Unified School District bus yard to install new glass on 79 buses damaged by vandals early Friday morning.

The buses are expected to be back on the streets Monday, said Dan Shearer, director of transportation for the school district.

The damaged buses were out of service Friday, but school was held as usual. The 19 schools whose bus routes were affected included all elementary, middle and high schools in the Arcadia, Coronado and Saguaro complexes. A total of 237 bus routes and 4,000 students were affected by the vandalism.

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Scottsdale police arrived at the yard around 2:45 a.m. after neighbors heard noises and reported a disturbance.

Superintendent John Baracy said 79 of the 80 buses in the yard near McDowell and Hayden roads had been vandalized. Scottsdale police Sgt. Mark Clark said the vandals appeared to have used a hammer or similar blunt object to smash the front and rear windows of the buses.

Police said two hammers were found in a nearby Dumpster. Lab analysis will confirm whether they were used in the vandalism.

Clark said the damage occurred between 1:30 and 2:30 a.m., during the few hours when the yard was deserted. Bus drivers usually arrive early in the morning to begin their routes and stay late into the evening, leaving a brief window for the vandals to strike.

Though the bus yard is walled and gated, Clark said there are no security guards because security had never been an issue.

Early damage estimates were pegged at $150,000, said Marijke J. Van Fleet of the Scottsdale Unified School District.

Clark said police were investigating whether the vandalism was related to a similar incident earlier this year. In April, vandals caused $35,000 damage by smashing the windows on 60 to 70 school buses in the same yard.

Police and district officials were working to notify parents of the affected bus routes. Baracy said an emergency parent notification system had been activated, and bus drivers told CBS 5 they planned to drive to the bus stops in their personal vehicles to inform parents of the situation.

Students and parents were asked to carpool Friday to reduce traffic congestion while dropping off and picking up children at the affected schools.