I don't think concern over skirt length should be discounted as merely "old-fashioned" thinking. Now, I'm all for individual expression with clothing, especially for teens. However, as educators we must take into account that this is the age that these girls are coming into their sexuality. It is our job to enforce limits, and the students' job to push them. Also, the line between child and woman is one of the most attractive to many (think how successful the little girl gimmick that Girls Generation or Orange Caramel puts out is), and makes them a greater target for harassment/attention that they are most certainly not ready to handle.
I think it's important that girls at this age are provided with a safe environment to explore and test out the limits of their sexuality, and I think it's natural that they will do so. However, I don't believe school is that place and I think it's better for educators to toe a more conservative line. If given no limits, I think these girls will actually have a harder time defining their own sexuality than if they were given limits. They don't fully understand their own sexual attraction to others or the import of making themselves look sexy, and its very easy for them to take it too far. I think there is a big difference between looking sexy and looking grown-up, but teens often don't understand that difference. As people who have gone through this stage ourselves, I feel that it's our job to help these girls understand more about it by pushing in the opposite direction of the one they are inclined to.
I think the desk idea is an utterly ridiculous waste of time and money. The best solution is to just get rid of uniforms altogether. I'm sure most of my girls would wear skinny jeans nearly everyday if given the chance.
I also don't think short skirts can be discounted as less sexy just because it's more acceptable here. It's more acceptable because looking good is a more important social value here and Korean women know the best way to accentuate a petite flat-chested figure with shorter, thin legs (which I'd say is the average body type here, while admitting many exceptions to it).