My 15 yo has several skirts that either skim the top of her knees or hit in the middle. None below. Do I need to run to Kohls?

They will probably be ok, but why risk it? Doesn't she own any pants or capris?

Oh right. So used to donning the dresses when she goes to mass that I completely forgot about that option.

In my experience a skirt that goes to the top of the knees is fine. The dress code is more about thigh and not knee caps. On a recent summer visit they were turning away women who were showing more thigh than was covered, bare shoulders, or 3" or more of cleavage.

Knee length shorts on men and women were fine.

YMMV.

In my experience, a skirt that does not cover the knees is NOT FINE.

My good friend, Heather, wore a skirt that touched the top of her knees to St. Peter's and the "fashion police" would not admit her. She had to jury rig two sweaters to make a longer skirt.

If you want to risk it, just bring along a pashmina scarf or something that wraps around her waist.

Thin

I've seen men with knee length shorts turned away also. Just depends on what mood the guard is in.

This may not apply to Rome but in all the Tuscan cities we visited earlier this month disposable capes and skirts (made out of what looked like "Handi-wipes" material) were being given out to both men and women with dress issues. The easy availability of these seemed to have led to slightly more strict rules than I remember from previous visits, but a very pleasant process for gaining compliance. Both men and women cheerfully wore their newly adapted garments in whatever arrangement was needed.

Some churches did not ask for the capes or skirts back. I would imagine that some travelers saved theirs for later. I almost wished I had needed one myself.

The Vatican has the "fashionista" police just outside the entrance to St. Peter's Basilica (this is AFTER you've already waited in line and gone through the metal detectors!). They will determine your fate. I've never seen anyone turned away for cleavage, tatoos, jeans, etc.... but if your knees and shoulders are not covered expect to be stopped.

We were in the Vatican Museums today, entering the Sistine Chapel, and the guard told a young lady in my group to cover her shoulders (she had on a sun dress and had removed her "shawl" due to the heat). I had never seen this before as it's often mentioned that there is no dress code for the Museums... obviously that's not correct and as kybourbon states, it's very "subjective" as to who they actually stop.