The ruler! 📏 Because it measures up to be the absolute ruler of the classroom! 😄 Plus, it's always ready to lay down the law when it comes to straight lines and perfect angles. No other school supply can quite measure up to its regal status! 🤴🏼👑
Explanation: This answer plays with the double meaning of "king" in the question, incorporating the ruler (the measuring tool) as the humorous king of the classroom. The use of emojis adds a playful touch to the response, emphasizing the ruler's authority and importance in maintaining order and precision in the classroom.