Goodbye, Mr. Chips

For the first time in 58 years, because of a cold, retired schoolteacher Mr. Chipping (Robert Donat) misses a first-day assembly at the Brookfield Public School. That evening he falls asleep in his chair and his teaching career is related in flashback.
When 25-year-old Charles Edward Chipping first arrives as a Latin teacher in 1870, he becomes a target of many practical jokes on his first day. He reacts by imposing strict discipline in his classroom, making him disliked but respected. 20 years pass, his relationship with his pupils improves and he becomes the senior master. He is disappointed in not receiving an appointment as a house master within the school for the following year. However, the new German teacher, Max Staefel (Paul Henreid), saves him from despair by inviting him to share a walking holiday to his native Austria.
While mountain climbing, Chipping encounters Kathy Ellis (Greer Garson), a feisty English suffragette on a cycling holiday. They meet again in Vienna and dance to the Blue Danube Waltz. This piece of music is used as a leitmotif, symbolizing Chipping's love for her. Chipping remarks that the Danube appears blue, but only to those who are in love. On another part of the same boat, as Kathy looks at the river, she notices that it is blue. Even though Kathy is considerably younger and livelier than Chipping, she loves and marries him. They return to England, where Kathy takes up residence at the school, charming everyone with her warmth.
During their tragically short marriage (she dies in childbirth, along with their baby), she brings "Chips" out of his shell and shows him how to be a better teacher. He acquires a flair for Latin puns. As the years pass, Chips becomes a much-loved school institution, developing a rapport with generations of pupils; he teaches the sons and grandsons of many of his earlier pupils.
In 1909, when pressured to retire by a more "modern" headmaster, the boys and the board of governors of the school take his side of the argument and tell him he can stay until he is 100, and is free to pronounce Cicero as SIS-er-ro, and not as KEE-kir-ro.
Chips finally retires in 1914 at age 69, but is summoned back to serve as interim headmaster because of the shortage of teachers resulting from the First World War. He remembers Kathy had predicted he would become headmaster one day. During a bombing attack by a German zeppelin, Chips insists that the boys keep on translating their Latin - choosing the story of Julius Caesar's battles against Germanic tribes, which describes the latter's belligerent nature, much to the amusement of his pupils. As the Great War drags on, Chips reads aloud into the school's Roll of Honour every Sunday the names of the many former boys and teachers who have died in battle. Upon finding out that Max Staefel has died fighting on the German side, Chips also reads out his name in chapel.
He retires permanently in 1918. He is on his deathbed in 1933 when he overhears his friends talking about him. He responds, "I thought you said it was a pity, a pity I never had children. But you're wrong. I have! Thousands of them, thousands of them â and all boys."

Question: What student behavior motivated Charles Chipping to imposing strict discipline in thie classroom?
Answer: Practical jokes