How much length you can recover is the depth of the existing turn-up minus the cloth needed for the new finish. Before you unpick both legs, open a small part of one hem and check the fabric that would show on the outside.
Inspect the old fold from both sides
Look along the ankle crease in daylight. A sharp line can be caused by pressing, colour loss or actual abrasion. Gentle flattening, within the label’s ironing directions, can show whether the line is mainly a crease. A shiny stripe, worn threads or a clear colour difference may still be visible after letting the trousers down. Decide whether that finish is acceptable before you commit.
Measure the usable cloth
Measure from the original fold to the inside cut edge. Work around the leg and note the shallowest usable point, allowing for fraying. Then work out the depth needed for the finish you can sew. If the turn-up has 35 mm of sound cloth and the new finish needs 20 mm, the example yields about 15 mm of extra length. A facing can be considered when there is too little cloth for a regular turn-up, but it still needs sound fabric to join to.
Put the new fold in place
On each leg, remove the old hem stitches without cutting the cloth. Mark the new finished edge on the wearer in school shoes. Fold and baste to that mark; check both leg openings for enough room over the shoes. If an old crease runs just above the new edge, look at it from the distance at which the uniform is normally seen. If you are satisfied with the length and appearance, secure the new hem using a finish suited to the fabric and remove the basting.
Know when the allowance is spent
Some trousers have already been let down or have a factory edge too narrow to move. If the new fold would run through frayed cloth, a neat result may need a different garment or an alterations service. Measure the second leg independently; a usable allowance on one leg says nothing about the other.
References
- New Mexico State University Extension — Making Perfect Garment Hems
- The Alteration Shop — school uniform repair and alterations