Class, Lass and Ass

Professor Tom was going to meet his students on the next day,so he wrote some words on the blackboard which read as follows: "Professor Tom will meet the class tomorrow."

A student, seeing his chance to display his sense of humor after reading the notice, walked up and erased¹ the "c" in the word "class." The Professor noticing the laughter, wheeled around, walked back, looked at the student, then at the notice with the "c" erased--calmly walked up and erased the "l" in "lass", looked at the flabbergasted student and proceeded on his way.