There is no picnic in Hilton’s novel. Chips I (a quiet conventional school teacher) meets Katherine Bridges (a New Woman) while traipsing the English Lake District. “One day, climbing on Great Gable, he noticed a girl waving excitedly from a dangerous-looking ledge. Thinking she was in difficulties, he hastened toward her, but in doing so slipped himself and wrenched his ankle. As it turned out, she was not in difficulties at all, but was merely signaling to a friend farther down the mountain; she was an expert climber, better even than Chips, who was pretty good.”
Katherine was not Chips type, but “within a week, they were head over heels in love.”
Picnicking begins with Sam Wood’s 1939 film that uses a stop on a hike for a picnic lunch. All subsequent versions of Goodbye Mr. Chips have a picnic. It’s a cinematic ploy to have Chips and Katherine meet, get chatty, and fall in love.
See James Hilton. Goodbye Mr Chips! Boston: Little Brown, London: Hodder & Stoughton, 1934. Illustrated by Ethel “Bip” Pares; https://gutenberg.net.au/ebooks05/0500111h.html#chap3: Sam Wood. Goodbye, Mr. Chips (1939). Screenplay by R.C. Sherriff, Claudine West, and Eric Maschwitz is based on James Hilton’s novel (1934).