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 to 1977 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 Enforcer (1976).