The most common and practical tool to measure the length of a classroom would be a measuring tape.

Here's why:

* Accuracy: Measuring tapes are designed for accurate length measurements, usually marked in feet, inches, or centimeters.

* Flexibility: They can easily bend and curve to measure around obstacles in the room.

* Portability: Measuring tapes are compact and easy to carry around.

While you could potentially use other tools like a ruler or a yardstick, they might be less practical due to their size and limited range.