Send Off Sir

A games master is red-carded at the annual school v. staff football match. His body is found shortly afterwards in the changing room, blood seeping from a wound to the back of the head. The police are uncertain whether the death was accidental or deliberate. The Senior Investigating Officer has an ulterior motive in assigning a particular policewoman to lead the investigation, which flounders due to her inexperience.
When a second teacher is found dead, the series' principal detective returns from secondment and takes over the two cases. The conclusion involves the resolution of conflicting moral dilemmas.