Do it in feet usually but do it in meters
Meter ruler is the metric tool that is used to measure a building.
Meters, or kilometers, depending on the distance. yards, or miles, depending on the distance.
it is yards or yd because the school does not stretch a mile
Meters or yards if it's the school library, in the same building or on the same campus as the classroom. Kilometers or miles if it's simply the municipal library, in the same city as the classroom.
time taking to walk
Every school is different. The layout of every school is also different. So you will need to find the distance from your classroom to the principal's office in your own school. Ask your teacher's permission to use a tape measure or yardstick to measure the hallway length between the two points.