The most important fashion rule is, in my opinion: do not dress for other people. That includes wearing something just because it's "cool" or "trendy" and wearing something the way another person likes it. But that also means dressing to feed the sexist norms of society and dressing the way other people want to perceive our bodies.
Stop telling girls that their dresses are short or too low. This goes for all adults. Trust me. We're well aware of how our clothing looks. More aware than you. When we stand in front of the mirror for 10 minutes every day inspecting ourselves and trying on different clothes, we know how it looks. If we leave the house in a piece of clothing, we have taken it off and put it back on and over-thought that outfit. If we're leaving the house, we feel comfortable in it. Do not comment. Don't do it. We're already self-conscious enough. Pointing out that our shorts are too short justifies our fears and is humiliating. If you see a bit more skin on someone than is necessary, just ignore it. Other people's bodies are none of our business!
9 times out of 10, I can assure you that girls do not wear "revealing" clothing to attract boys. It's because it looks cute and that is all. I hate the term "slutty" when it comes to clothing. It's like, oh, I'm sorry…I didn't know the length of my skirt was a direct indicator of the number of sexual partners I've had!
Then again, we're not idiotic. We're not going to wear a super short skirt to a job interview. Is there anything wrong with a short skirt? No! I sure hope the person hiring wouldn't judge you on the length of your skirt, but most short skirts are for daytime fun and not job interview appropriate. The same thing goes for religious establishments- even if the modest clothing a religion practices does not match up with your personal beliefs, it's best to respect those beliefs if you're visiting a church/temple or going into a religious family's home.
But life's not a job interview and a single belief does not dictate humanity. There's a time and place for modest clothes, but daily life does NOT confine you to those kind of outfits. If you like a piece of clothing and feel great in it, then there is NOTHING wrong with it. It doesn't matter what anyone else thinks. And if you're on the other side: someone's body and clothes are not your business. Do not comment.
You do not dress for other people.