The city school bus drivers union announced today that its members would strike starting Wednesday over the city's plan for new contracts with bus companies.

After more than a year of strike threats, city school bus drivers will walk off the job on Wednesday, their union announced today.

The work stoppage means that more than 150,000 students — including many with severe disabilities — will have to find their own way to school. All students affected by the strike who can get to school using public transportation will receive Metrocards, and the city will reimburse families who must drive or hire cars for the commute to and from school.

Still, city officials say they expect that the burden of providing transportation will lead at least some families to keep their children home.

The strike also means that the city’s streets will be clear of yellow buses for the first time since 1979, when the city ended a three-month strike by extending new protections for drivers.

The strike comes as the city prepares to seek contracts with bus companies in an effort to cut student transportation costs, which are the highest in the country. The drivers’ union, Amalgamated Transit Union’s Local 1181, wants a guarantee that current employees won’t lose their jobs even if the companies they work for do not win a new contract. But the city, citing a 2011 legal ruling, says it cannot make such a promise.

“Have you ever heard of a strike where one side is demanding something that the courts have ruled illegal?” Mayor Bloomberg said today during a press conference just before the union officially declared the strike. “It is just meshugana, as we say in Gaelic.”

Labor leaders from across the city state are arguing that the law isn’t as fixed as Bloomberg has portrayed it. “This is a completely avoidable situation that the city could solve in an instant if it only had the willingness to do so,” New York City Central Labor Council President Vincent Alvarez said in a statement.

But for the union to reconsider the strike plan before Wednesday, Local 1181 President Michael Cordiello said, the city’s change in position “would have to be meaningful.” He said the strike would last “until the mayor helps us.”

But Bloomberg said city officials were prepared to dig in for the long haul.

“They should not be lulled into thinking we’ll change our minds,” Bloomberg said. “We will go and ask for bids, and in the meantime we’ll find ways to deal with [the strike] that are not unsatisfactory.”

The city’s stop-gap solutions include giving Metrocards to students who currently ride yellow buses; offering Metrocards to the parents on the youngest bus riders; having additional safety officers direct traffic at some schools; reimbursing transportation costs for families that front the bill; and warning the Metropolitan Transit Authority to expect extra congestion on subways and city buses.

Even so, the Department of Education is anticipating that at least some students who ride yellow buses won’t be able to get to school easily starting on Wednesday. Students who are less than two hours late to school won’t be marked tardy, and absences because of transportation issues won’t count against students’ academic records.

Some schools have asked teachers to prepare work for students to complete at home. At Brooklyn’s P.S. 231, a District 75 school for students with severe disabilities where virtually all students take yellow buses, teachers were told earlier this month to compile 10 days’ worth of assignments that students could take home in case of a strike, one teacher reported on Twitter.

“A lot of parents won’t be able to get their kids to school, or will miss a lot of work,” said Susan Valdés-Dapena, a Queens parent whose son rides a bus to his private school on Roosevelt Island, which he attends using state funding for students with disabilities.

But Valdés-Dapena said she was supporting the bus drivers anyway because her son, Patrick, has benefited from having experienced bus drivers.

Patrick Valdés-Dapena said he had a different reason for backing the strike. ”I am a little bit excited about the strike,” he said. “It sort of seems like we’re not going to have school.” (His mother disagreed: She said she would drive Patrick to school.)

The bus drivers strike isn’t the only labor issue the Department of Education is juggling right now. The strike is set to start the day before a state teacher evaluation deadline that has had the city locked in negotiations with the teachers union, as well.

“We’re working on them simultaneously,” Lauren Passalacqua, a City Hall spokeswoman, said about the teacher evaluation and bus contract negotiations. “Both issues are priorities and it’s a balancing act.”