The district probably hired teachers to run the ITV service, and/or got permission from Britannica and other copyright holders to run 16mm films here and there. For the most part, instructional television largely went away by the mid 1990s as most schools had an educational VHS library that teachers could borrow. That has now been replaced with YouTube.
Instructional TV did come back (briefly) in some places during the COVID shutdowns in spring 2020. KICU 36 in San Jose ran an hour or two of SFUSD programs daily, as did WMC-5 in Memphis on their Bounce TV subchannel. Some PBS stations also aired instructional programming much like the old days. But I'm sure some of them were in shoddy quality as they were taped in teachers' homes.