Keep the folded cloth inside the leg if the trousers may need letting down later. On straight woven school trousers, you can often shorten the leg without cutting anything. Try the new fold first: if the turn-up is too bulky or too tight inside a tapered leg, it needs a different finish.
Mark the finished edge first
Have the wearer try on the trousers at their usual waist height, with school shoes. Mark both legs using the separate measuring guide below. The finished edge is where the fabric bends back on itself. The allowance is the cloth that extends beyond that bend on the inside. A cut made on the finished-edge line leaves no allowance to secure the hem.
Try a reversible fold
Open the old stitching without pulling threads from the trouser fabric. Flatten the allowance and fold the leg to the new mark, with side and inside seams aligned. On a small adjustment, the old edge may sit flat inside the new hem. Baste around the leg with temporary stitches and look at it from the outside. A hard horizontal ridge, twisting seams or a turn-up that has to be stretched into place means the fold is too bulky or its widths are mismatched.
Work out what would be left if you trim
For example, suppose the existing edge is 45 mm below your new fold and the fabric can take a flat 30 mm allowance. Mark the proposed cut 30 mm below the new fold; the offcut would be 15 mm. These are example measurements. Check your own allowance around the whole leg, because the old cut edge may slope. Trim only after the fitting, only if keeping the extra cloth makes the result worse, and only with one fabric layer under the scissors. Finish any raw edge before sewing.
Check the result on the wearer
Use the original type of finish if it suited the cloth. Sew through the turn-up and the intended layer only. With clips removed, check the trousers while standing and walking in the shoes. Compare both sides at floor level. Record how much fabric is left inside each hem for the next adjustment.
References
- New Mexico State University Extension — Making Perfect Garment Hems
- The Hills Shire Council — clothing and textile repair demonstrations