Picnic at Hanging Rock

At Appleyard College, a girls' private school, near the town of Woodend, Victoria, Australia, the students are dressing on the morning of St. Valentine's Day, 1900. Miranda (Anne-Louise Lambert), Irma (Karen Robson), Marion (Jane Vallis), Rosamund (Ingrid Mason), waifish Sara (Margaret Nelson), and outsider Edith (Christine Schuler) read poetry and Valentine's Day cards.
The group prepares for a picnic to a local geological formation known as Hanging Rock, accompanied by the mathematics mistress Miss Greta McCraw (Vivean Gray) and the young and beautiful Mlle. de Poitiers (Helen Morse). On the authority of the stern headmistress Mrs. Appleyard (Rachel Roberts), jittery teacher Miss Lumley (Kirsty Child) advises Sara that she is not allowed to attend.
Driven by buggy operator Ben Hussey (Martin Vaughan), the party pass through and arrive at the Rock by mid-afternoon. After a meal, Mr. Hussey notes his watch has stopped at the stroke of twelve, as has the watch of Miss McCraw. With permission from Mlle. de Poitiers, Miranda, Marion and Irma decide to explore Hanging Rock and take measurements, with Edith allowed to follow. The group is observed several minutes later by a young Englishman, Michael Fitzhubert (Dominic Guard), who is lunching at the Rock with his uncle Colonel Fitzhubert (Peter Collingwood), aunt Mrs. Fitzhubert (Olga Dickie), and valet Albert (John Jarratt). At the top of Hanging Rock, the group lies on the ground, apparently dazed by the sun. Miss McCraw, still at the base of the Rock, stares up. Miranda, Marion, and Irma awake and move, as if in a dream, into a recess in the rock face. Edith screams and flees down the Rock.
The distraught and hysterical party eventually returns to the College, where Mlle. de Poitiers explains to Mrs. Appleyard that Miss McCraw has been left behind. Sara notes the absence of Miranda; and Mr. Hussy explains to Mrs. Appleyard that Miranda, Irma, Marion, and Miss McCraw went missing. A search party, led by Sgt. Bumpher (Wyn Roberts) and Constable Jones (Garry McDonald) of the local police, finds nothing, although Edith reveals that she witnessed Miss McCraw climbing the Rock without her skirt. Michael Fitzhubert is questioned and reveals he watched the schoolgirls but can provide no clues as to their whereabouts.
Michael becomes obsessed with finding Miranda; and, with Albert, he conducts another search of Hanging Rock. Despite Albert's protests, Michael decides to remain overnight and begins climbing again the next day, leaving a trail of paper. When Albert follows the markers, he finds a nearly delirious Michael. Just before leaving on a buggy with a local doctor, Michael passes to Albert a fragment of lace from a dress. Albert returns to Hanging Rock and discovers Irma, unconscious but alive. The residents of Woodend become restless as news of the discovery spreads. At the Fitzhubert home, Irma is treated for dehydration and exposure, and tells the police and Mlle. de Poitiers she has no memory of what happened. A servant notes that Irma's corset is missing but is advised by Mrs. Fitzhubert that it is not important.
Michael befriends the recovered Irma but alienates her when he demands to know what happened on the Rock. Mrs. Appleyard advises Miss Lumley that several parents have withdrawn their children from the school. Before leaving for Europe, Irma visits her classmates a final time; but they become hysterical and demand to know what happened to their missing friends. Mlle. de Poitiers intervenes; and, as Irma flees, she also notices that Sara has been strapped to a wall by Mrs. Appleyard to correct her posture. That night, Miss Lumley gives notice to a drunken Mrs. Appleyard that she is resigning.
Mrs. Appleyard tells Sara that, as her guardian has not paid her tuition, Sara must return to the orphanage. Afterwards, Mrs. Appleyard lies to Mlle. de Poitiers and claims that Sara's guardian collected her early that morning. The next morning, Sara's body is found in the greenhouse by Mr. Whitehead, the school gardener. Believing Sara committed suicide by leaping from her bedroom window, Whitehead confronts Mrs. Appleyard, who is calm in full mourning dress with her possessions packed. Michael tells Albert he has decided to travel north, with Albert revealing he had a dream in which his lost sister Sara visited him.
During a flashback to the picnic scene, Sgt. Bumpher states in a voice over that the body of Mrs. Appleyard was found at the base of Hanging Rock and that the search for the missing school girls and Miss McCraw continued sporadically for several years without success. Their disappearance remains a mystery.

Question: Who is not allowed to attend the picnic?
Answer: Sara