In the San Francisco police department, an "Inspector" is just a detective rank. In other police departments, an Inspector is a much higher rank (in the NYPD, Inspector is the rank above Captain; in the UK police, an Inspector is equivalent to a Lieutenant).
Following the success of producing One Flew Over the Cuckoo's Nest (1975), Michael Douglas left the series at the beginning of the 1976-77 season. His character's absence was explained by having him take a teaching position. Inspector Keller taking a teaching position is similar to a scene in Dirty Harry (1971) where an SFPD Inspector injured in the line of duty stated that he had a teaching credential and later became a college professor (which was stated in the sequel, Magnum Force (1973) as well as the third sequel The Enforcer.