Question about Mormon modesty?

Considering there was a mormon girl who went to my high school I would say from the neck to the feet. This girl wore long dresses and pretty, unrevealing blouse shirts. In gym class she got into trouble cuz we had gym uniforms we had to wear (school shorts and tshirts) and she would only wear her gym shirt and wore he long dress. The gym teachers got mad at her and gave her detention cuz apparently you cannot run in a long dress. However, she told her parents and called the principle and told them that they were mormons and said the school is discriminating against their religious freedom, needless to say the gym teacher took the detention away and let her wear her long dress to gym class. I asked her why she wore the dress cuz it looks like it was hard to run in. She told me her religion requires her to wear it for modesty. I told her it was just the bottom of her legs that would show in the school shorts, not like its your breasts and she said legs are immodest. I was like ah ok O.O