BROOKLYN - The union that represents the city's school bus drivers is threatening to go on strike beginning the day students return from Christmas break.

The Local 1181 of the Amalgamated Transit Union represents more than 9,000 school bus drivers. The union is threatening to strike because the city is bidding out thousands of school bus routes.

Drivers say they want assurances that their jobs will be protected based on seniority amid the new contract bids. As of now, the city has not agreed to that.

The strike became more likely on Friday when Mayor Michael Bloomberg moved forward with a plan to reign in the $1 billion expense of busing kids to school by putting the 1,100 bus routes now covered by 52 private companies out to bid.

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