I was amazed when the OP was worried that 6" above the knee might be obscene, so I tried to Google it, but everything that came back was about school dress codes!
FWIW, those varied between 1" and 6" above the knee as a minimum length, and those few that specified a range including a maximum length ALL said the same range, which was 4-6" above the knee (that is, except for one that said the hem musn't scrape on the floor!). From my own schooldays, I think hems had to be in the range of between 3" and 5" above the knee (I was crestfallen when they allowed the girls to wear trousers!), although not being a girl I cannot be sure if those are the right numbers, and I know that many of the girls wore skirts that were both shorter and longer than they were supposed to be.
Anyway, that's all rather OT except that I'm pretty sure that 6" above the knee isn't considered obscene when there are apparently plenty of schools that have put in writing that schoolgirls can wear that length, bearing in mind that they are supposed to be in loco parentis (look it up). Maybe not in YOUR locality, but that's another story.
Of course, that length might be considered obscene for a man in a kilt, for various reasons too delicate to discuss, but that's another matter altogether!
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