The Bloomberg administration set in motion on Friday a broad contingency plan for a school bus drivers’ strike, sending letters to parents and principals and buying hundreds of thousands of MetroCards for students who could be left without transportation. But a statement by the union’s president indicated that the timing of a strike, if one were to be called, was unclear.

A strike would affect more than 150,000 New York City public school students, from preschool through 12th grade, including about 70,000 children who receive special education services.

In his letter, the city schools chancellor, Dennis M. Walcott, called the strike a “strong” and “immediate” possibility. But the bus drivers’ union, Amalgamated Transit Union Local 1181, offered no clues about its timing. In a statement, its president, Michael Cordiello, said that a strike was likely but that “there are no immediate plans for one.” The union’s members are expected to discuss the strike, the threat of which is centered on job protections for the union’s most senior members, at a meeting on Tuesday.

Nonetheless, the city spared no effort to plan for the event, drawing up detailed rules, conditions and safeguards. Field trips requiring yellow-bus service would be canceled, but after-school programs would continue. Delays of up to two hours would be forgiven, and absences would be excused from students’ attendance records if they happened as a result of the strike.