Place the patch behind the hole so it sits against the inside of the leg. The patch needs to extend beyond the worn halo around the hole; stitches close to a fraying edge can tear out. This works best on woven school trousers with a localised tear and firm fabric around it.
Map the sound cloth
Lay the leg flat and look at the knee in good light. Mark the edge of the area where the weave is thin or distorted. Choose a patch of similar weight and stiffness, and align its grain with the trouser leg so the fabrics behave in a similar way. If the trousers can be washed, use patch cloth that has already been laundered in a compatible way to reduce uneven shrinkage. Avoid a thick patch that makes the knee hard to bend.
Place and hold the patch
Cut it large enough to reach solid fabric on every side. Round the corners if that makes it easier to keep the edge flat, and finish a raw patch edge if it frays. Insert a firm piece of card between the layers of the leg and baste the patch into place from the inside. Check from the front that the tear lies in its natural position, without drawing it shut so tightly that the knee puckers. Remove the card before using a machine.
Sew where the weave can carry the stitches
Secure the outer part of the patch to sound cloth with a method you can control. On a narrow leg, hand sewing can be easier than fitting the leg around a machine’s free arm. If loose threads around the hole lift away from the patch, stitch the sound edge down or stabilise the frayed strands with short stitches into the backing. Leave space for the knee to bend; do not tighten the thread enough to gather the fabric.
Check from the inside and outside
Remove the basting. Slide a hand through the leg, bend the patched area and check that the corners stay flat. If there is no firm cloth to reach before a seam, or the knee tears again next to the patch, consider a larger professional repair. For school trackies, use the stretch-fabric guide because the patch has to move with the knit.