Why did the ruler feel important in class?

This joke hinges entirely on a clever bit of wordplay, specifically a pun. The humor comes from the double meaning of the phrase "measured up." In one sense, a ruler literally "measures up" objects by providing precise lengths and dimensions in a classroom. But the punchline cleverly twists this literal action into the idiomatic expression "to measure up to expectations," which means to meet or fulfill certain standards or hopes.

The real-world context here is straightforward: rulers are ubiquitous tools in schools, used for everything from geometry lessons to art projects. They are fundamental for understanding concepts of length and proportion. The phrase "measuring up to expectations" is also a common idiom we use to describe performance, whether it's a student's grades, an athlete's achievements, or even a new product's success. The joke plays on our familiarity with both the physical object and the common expression, creating a lighthearted connection between the two.