The City filed a charge with the National Labor Relations Board in an attempt to head off the threatened drivers' strike.
The dispute stems from a bid for a new contract for pre-K buses recently put out by the education department.
The bus drivers' union has told the city it will go on strike if the city doesn't commit to job protections for current bus drivers as it looks to secure a new yellow-bus contract.
The current contract is set to expire at the end of June 2012
Schools Chancellor Dennis Walcott sent a letter to families saying, "We are deeply concerned about the impact of a strike on our students and families and we want you to be prepared in the event one occurs."
Chancellor Walcott says the city believes the strike would be illegal and is taking steps to get a federal court injunction preventing it.
The union says the drivers are not planning an immediate walkout.
"The issue here is getting children to school safely and securely. All the Mayor has done is create more chaos, instability, and concern among parents about NYC school buses, which have already been poorly managed for years," the union statement read.
A strike could impact more than 150-thousand school children.
"If and when a strike should happen, we are going to do everything possible to help parents who rely on school buses to get their children to school safely," Mayor Bloomberg said.
The Education Department is forming a contingency plan that includes:
The schools system will also give a two-hour reprieve to children who arrive late in school because of transportation problems.
More information is available at nyc.gov.