Hundreds of Boston school bus drivers struck unexpectedly Tuesday morning, leaving students and parents looking for alternative transportation. Some children were delivered to school by police car, The Boston Globe reported. Police officers cruised the streets looking for stranded students. While the schools were open, parents and students were told there would be no penalty for students who were late or absent because of transportation problems. The strike was called by United Steelworkers Local 8751, which represents the drivers. Steve Kirschbaum, chairman of the grievance committee, said that since Veolia Transportation Inc., based in Illinois, took over the transportation contract for the Boston Public Schools, there have been problems with work rules and unilateral changes in their contract. About 700 drivers are represented by the union. Only 30 of the 650 buses were on the road Tuesday morning. Officials called for an immediate end to the walkout. \"Thirty-three thousand students and their parents have been affected today as a result of this illegal work stoppage by the bus drivers\' union,\" Mayor Thomas M. Menino told reporters at a news conference. \"The bus drivers, let me tell you, have a contract. A very good contract. Today\'s actions are by some angry people who don\'t follow the rules.\"