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