New York City Public Advocate Bill de Blasio on Tuesday urged Mayor Michael Bloomberg to bring all parties of the school bus strike together for a "cooling off" period and to get drivers on the road again.

"The absence of these buses has changed the lives of our children for the worse," de Blasio said, holding a letter signed by parents asking the mayor to step in.

De Blasio, a Democrat running for mayor, said the strike is a "conflict that didn't have to happen" and cited precedents in which mayors have stepped in to end labor disputes. The school bus drivers union, Local 1181 of the Amalgamated Transit Union, has been on strike since Jan. 16, with no end in sight.

Nearly a third of disabled students have been unable to get to school since the strike began, de Blasio said.

"The mayor is acting like a bully in the schoolyard," said Councilwoman Melissa Mark-Viverito (D-East Harlem), who stood with de Blasio outside City Hall with parents and children. "He's holding the transportation and education of our children hostage for politics."

The mayor's office responded the city needs to put out the bids for the school bus operators to bring down costs. The union seeks security for drivers in companies that don't win contracts. But the city contends the issue is something "we legally cannot include . . . and hiring issues are not something the city can dictate to a private employer and their employees," a spokeswoman said.

"It's sad that special interest advocate de Blasio is siding with adults who are using children as pawns," said a Bloomberg spokeswoman. "If he [de Blasio] wants to help kids, he should persuade the union to return to work."

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Joe Williams, whose 14-year-old son is autistic, said it takes nearly four hours daily to get his child to and from school.

"He's missing valuable instruction time," Williams said. His son receives speech, physical, and occupational therapy. "This is affecting our children more than just academically. It's affecting them physically, mentally, and psychologically."