I recall seeing this mystery film in the 80s, possibly on PBS, perhaps on MYSTERY! Then again it may have been a rental tape.
At any rate, it involved a murder investigation being conducted at a British boys school. In the only scene I can remember, one of the detectives went into an attic storage space, only to have the door pulled shut, locking him in while a fire was set down below, filling the room with smoke.
I seem to recall one of the investigators resembling a young Terence Stamp, but I’ve been through his filmography with a fine tooth comb and it doesn’t seem to have been him.