Funding and enrollment growth are the reasons that districts have had to keep these buses. Washington has a nationally recognized school bus inspection program performed by the Washington State Patrol on all buses in the state annually. Washington has a nationally recognized driver-training program. Washington has an excellent safety record, which has probably worked against increased funding from the legislature to replace the pre-77 buses. They haven't seen it as a problem.
Kids are safer going to school on a school bus than by any other means as shown below.
48,000,000 Public School Students Enrolled 24,000,000 students transported in school buses with 12 fatalities . 24,000,000 students not in school buses with 799 fatalities .