When letting down pants or school uniforms, you are usually left with a fold line from the original hem.  There are a number of things you can do to eliminate the crease.

1.  Unpick the hem and mark the crease with soap on the front and back then wash

2.  Attach a false hem and stitch through the old crease line putting two rows of stitching.  This usually covers it.

Happy sewing

Judith aka genie