My view is probably different because I went to an all girls' very southern school, but I'll give my input.
They used to just specify that it had to be white nice formal clothes. However, one year one girl wore pants, there was an OUTRAGE from the board of trustees and older parents/grandparents, so they began to specify that it HAD to be a dress or skirt, all white. My mother refused to let me try anything and forced me to get a white dress. I blocked out the experience, I honestly cannot for the life of me remember getting the darn thing, but I did at some point. I just changed into pants/shirt as soon as I took the gown off. However, it clearly wasn't THAT important because there was another (girl? Idk what their identity is, so I hesitate to say girl) who walked and wore a dress shirt and white slacks, and I didn't hear anyone complaining. They were allowed to walk and everything. And its not like its super noticable, after all, you're wearing the gown. Unless they've threatened to not let you walk, I think you can go for it, and honestly if you're not allowed to walk, is it really a big deal? I know at my school it was a HUGE deal to not walk because people only didnt if they got in big trouble, but at other schools I know its not exactly abnormal to skip it.