A Boston public schools spokesman says about 600 school bus drivers have gone on strike, leaving thousands of children without a way of getting to school.

The school system says the strike Tuesday by the United Steelworkers Local 8751 is illegal.

A telephone message could not be left at union headquarters because the voicemail box is full. An email message was left.

The drivers are not school employees, but work for the city's bus contractor.

The spokesman says the strike was prompted by the union's opposition to changes that ensure driver safety, improve on-time performance, and a web tool that allows parents to track buses in real time.

Most of the district's 33,000 students are affected.

Schools will be open, but students will not be marked late and absences will be excused.