Teen, the knees are supposed to be covered. the “4 inches” rule was that someone figured out if s girl sits, 4 inches would be enough extra length to ensure that the knees remain covered. It’s not exact, of course.
The Oz vehadar levusha has many chumras in tznius that are not me-ikar hadin. The knee covering is, though. If the school tells your sister she can’t wear sneakers, then you have a complaint.
there is a lot of am haaratzus out there. Many people don’t have rabbeim, and do what everyone else does. Many schools have adopted ridiculous tznius standards to prove that they are the best. Find a competant rov who actually knows halacha and the sources for any questions you may have. “Oilem Goilem”, meaning the world is full of people who have no clue.