My guess is that it's too casual for the non-casual days, but Friday would probably be ok.
Is your office air-conditioned? If so, it shouldn't be a big deal to opt for longer skirts during the summer. I personally prefer the longer ones. No worries about showing too much leg--and honestly, I'd rather be able to focus on the task at hand, not in pulling down my skirt or adjusting other clothing.
Q: How many poets does it take to change a light bulb? A: 1001...one to change the bulb, 1000 to say it's already been done.