A microskirt is a very short skirt indeed. It is shorter than a miniskirt, being less than 20 cm (8 inches) in length. They are predominantly worn by teenage girls or young women to evoke an impression of cheekiness and playfulness, especially in an appropriate social context.
Japanese student with ‘kogal’ loose socks and skirt
Microskirts are rarely worn as streetwear in Europe.
They are, however, quite popular among girls in Japan, where they are made part of school uniforms.
The uniforms grils wear to school in Japan always include a skirt which is made shorter by the ‘popular girls’ within the kogal subculture – the culture of popular youths.
Microskirts are important within kogal and the fashion item is popular among teenage girls, especially among those who want to practise panchira, a form of softcore exhibitionism where women try to show just a hint of their panties.
Nowadays the term panchira is also used by Japanese women to warn each other that their underwear is visible.
Knowledge Guild 
Wow, that is so weird… I try to wear skirts that get to my knee, no higher. I leave things up to the imagination, and I like it that way!