Seems like you are going to fight a battle for something that won't affect you. If you do not intend to dress at work then this potential policy only bothers you because you think it "might" be the result of your wearing heels and skirt to an event last year. You most likely would never be able to substantiate that, so take it easy on yourself. Many occupations have dress codes so I do not think other women drivers wear skirts will have to "suffer" and simply wear pants. Most female uniformed bus drivers and transit workers I have seen wear pants.
Maybe no one else will really care. Maybe there are other reasons for the policy change. Maybe there were complaints from parents when you wore skirt and heels to the ceremony last year and the school board had to do something. Maybe your union should push to get everyone a clothing allowance or uniforms if new dress codes are necessary. Maybe ...maybe...maybe...
By the way, it took balls to wear the outfit you did to an employee of the year event. Wow! You obviously are good at your job for being nominated. You clearly have guts. Just try to ease your way out of responsibility for something that might or might not happen. Congratulations for your nomination.