Absolution (1978)

At a Catholic public school, Benjamin "Benjie" Stanfield (Dominic Guard) is tired of being the teacher's pet and decides to play a practical joke on his form master Father Goddard (Richard Burton). In confession, Stanfield tells Goddard that he has accidentally murdered his friend Blakey (Sir Billy Connolly) and buried him in the forest. When Goddard investigates the matter, he finds a buried scarecrow. Goddard is outraged, but, due to the seal of confession, he knows he cannot expel Stanfield. Shortly after, Stanfield once again enters the confession booth, telling Goddard that what before was a practical joke, he has now made happen. In disbelief, Goddard once again goes to the forest to investigate the matter. This time, he discovers Blakey's dead body. The plot soon thickens as Stanfield's fellow student Arthur Dyson (David Bradley) mysteriously disappears.

IMDb: https://www.imdb.com/title/tt0081975