I know plenty of teachers that refuse to wear dresses or skirts in the classroom. The fear is that sometime during lessons, the hem is going to ride high. There is a way to ensure this doesn't happen to you (see above). The goal is to have your hemline at a flattering, yet acceptable level. I prefer to only wear knee length or longer during the hotter months and only wear knee or above the knee lengths with tights and boots during the colder months.
This picture of Rachel Bilson shows the look that I like to go for during warmer months. I might switch the blazer out for a lighter cardi if it is too hot and sticky.
http://www.thestyleandbeautydoctor.com/wp-content/uploads/2011/05/rachel-bilson-at-derek-lam.jpg