Delegates from the city teachers union in Chicago overwhelmingly approved a tentative contract agreement, averting a midnight strike that would have idled 409,000 students in the nation's third-largest school district. The pact calls for a 3% pay raise in February followed by a 7% raise in 1993. The raises would cost the district $118 million next year. The school board previously projected a $178 million loss in the next school year. The agreement came after an around-the-clock negotiating session mediated by Mayor Richard M. Daley. During previous talks, the teachers' contract had been extended several times.