Thigh high boots are the absolute hotness. What makes them classy or trashy is the quality of the boot, along with what you are wearing with the boot. Outside of the bedroom, do not wear shiny pleather boots. Save the patents and pleathers for the bedroom. Go for a more matte finish, and make sure you layer your clothing. Meaning, it's good to wear a pair of leggings with a nice tunic, belt, jacket, scarf and or necklace. That gives you a layer of sophistication that does not scream hooker.
I do not recommend mini skirts unless you have leggings on underneath and have layered your top (see note above).
Doing this creates a harmony throughout your ensemble and makes you look chic instead of trashy.