California’s schoolkids are breathing unhealthful exhaust spewed by diesel school buses that are among the nation’s oldest and most polluting, according to a report being released today by the Coalition for Clean Air. The report urges the administration of Gov. Gray Davis (D) to set tough emission standards for school buses and to provide tens of millions of dollars to help school districts replace their fleets with new buses powered by cleaner-burning alternative fuels. Officials from the California Air Resources Board and the Los Angeles Unified School District agreed yesterday that the current bus fleet poses an environmental threat to children, but they have yet to decide on strategy to deal with the problem.