My mother-in-law told me she wore mini-skirts to work in the 60s. i asked her how did she keep her modesty when bending over, walking up stairs. She said the ladies managed. It was just the fashion at the time.
Pretty much, unless you were super conservative or a granny, you wore the minis. Modesty could be tricky but you learned how to move and reach and bend, etc. My personal trick in the winter at school was to keep my maxi coat (in style the same time that miniskirts/dresses were) on while reaching up into my locker to get my books.

I don't remember women flashing their undies unless they did it on purpose. And unlike today with thongs, the young ladies wore bikini undies. If you wore a micro-mini, more often than not, it came with panties made of the same material.
Plus back then you did NOT wear a dress/skirt without pantyhose. Some women wore undies beneath them, others did not.
I can't vouch for going up the steps as I wasn't looking at women's butts. I wasn't worried about flashing my undies when going up or down our school steps (our school was two-story) but was worried more about falling down the steps in the platform heels. I almost did once.