A contract stalemate continued in Chicago, leading officials to cancel classes today. A school board spokesman said the two sides would meet later in the week with a federal mediator. He said the nation's third-largest school district is unable to provide the 15% raise teachers want over a two-year contract. The strike forced cancellation of the start of classes for the city's 430,000 public school students last week. Meanwhile, strikes continued in six other states, with only one small district in Michigan reporting a tentative settlement.