Mark each trouser leg on the wearer before doing any cutting. Two identical measurements along a seam can still look uneven once the trousers are on, especially if one leg twists or the waistband sits differently from side to side.
Set up the fitting
Use the shoes and belt, if any, that normally go with the uniform. Pull the waistband into its usual position and allow the cloth to settle. Have the wearer stand on a level floor, looking ahead with weight across both feet. Read the school’s uniform guidance if it specifies length. Choose a finished position that clears the floor and still works when walking.
Mark a level line rather than copying a raw edge
A helper can hold a ruler vertically from the floor. Take a reference reading at the chosen front point, then check the side and back of the same leg. Use chalk or clips to mark the finished fold. Repeat on the other leg, using the same floor reference to begin with. Keep the leg hanging freely between marks; pulling down on a cuff while measuring can create a false reading.
Compare the marks on a table
Once the trousers are off, check how the new line meets each seam. If one mark sits conspicuously higher than its neighbours, check the fitting again instead of connecting the dots in a jagged line. On each leg fold at the marks and loosely baste. Label the left and right leg if the marks differ. You may want the finished lengths to differ slightly if that is what makes them sit evenly when worn.
Make a second fitting your checkpoint
With the basted folds in place, check the front, side and back while standing and taking a few steps. Make sure the back hem stays clear of the floor. A reading such as “25 mm from the floor” is a fitting note for that pair and shoe combination; it does not replace the wearer’s fit check. Once satisfied, use the separate shortening guide to work out the cutting line, if one is needed.