A teacher friend injured his back and had to wear an upper-body plaster cast.
At the start of the new term, with the cast still concealed under his shirt, he was assigned the school's most undisciplined class. He stepped confidently into the rowdy classroom and opened the window wide. A sudden gust of wind made his tie flap. To secure it, he picked up a blackboard eraser and hammered a large tack right through his tie and into his chest.
He had no discipline problems for the rest of the term.