Who put itching powder in the headmistress's gym knickers?

In chapter 10, "Throwing the Hammer," Matilda's older schoolmate Hortensia tells Matilda that she bought an itching powder called the skin scorcher—"made from the powdered teeth of deadly snakes"—and sprinkled it into all of Miss Trunchbull's knickers. It worked perfectly. One day in assembly, Miss Trunchbull began itching herself "like she had a wasp nest down there" and had to run out in the middle of the Lord's Prayer.
Though it is impossible for Miss Trunchbull to know exactly who the culprit was, Hortensia warns Matilda that Miss Trunchbull "has a nasty habit of guessing." A few days later, Miss Trunchbull grabs Hortensia by the ear, and ignoring her protests of innocence, throws her straight into the chokey. At the end of Hortensia's story, Matilda astutely observes that "It's like a war."

"You're darn right, it's like a war," Hortensia cried. "And the casualties are terrific. We are the crusaders, the gallant army fighting for our lives with hardly any weapons at all and the Trunchbull is the Prince of Darkness, the foul serpent, the fiery dragon with all the weapons at her command."