I'm sure the supplier can accommodate to provide a skirt suitable for the student in question, if asked, but I don't think that's the solution the school wants to arrive to. Also, it's weird to see a student being punished because her skirt was too long...
Yea, this seems to be the principal issue here. There's nothing remotely controversial about long skirts so they should be able to approve some just fine. I mean they approved pants, so surely they are realizing some people want to cover their legs. And the vendors will sell whatever, they'll be glad to make more money from more merchandise.