Glad to help, Dojo. Leggings-under-skirt is an inexpensive solution.
I completely concur with Blondie on writing to the headmaster. And it might be best to turn away any student who doesn't conform to the lesson "dress code" for the day.
Anecdote: Reasons for weird sitting positions.... when I was a teen in high school in the early '70s, we got driving lessons sponsored by the school. The teacher -- an older man -- made all students drive with the left knee to the door (I'm in the States). We learned on an automatic transmission. We girls found it a bit creepy, but we had pleated uniform skirts, were covered, and he never did a thing to us... years later I drove a manual transmission exclusively, till one day I was forced to drive an automatic again, and sat utterly befuddled for moments -- my feet expected a clutch! That high school memory came to mind and I put my left knee to the door: Voila! in that position, I wasn't inclined anymore to use my left foot for the non-existent clutch, and drove easily. Today, I suspect that driving teacher would have been reported and chastised...
2015 Connor spruce/Indian rosewood
1978 Ramirez 1a cedar