Given the pretty detailed rules on dancing, I can see that rules enforcement is taken seriously at these events. Based on that contract I am going to take the unpopular view that the kilt is already banned from the event. This is not to say you will be unsuccessful in reversing that ban, but as others have said be prepared for disappointment.

Some sections that stuck out to me in the contract were these:

Must wear collared shirts, slacks and tie apparel.
Any other clothing or dress deemed inappropriate by the Mission Viejo High School Administration.
Based on this it would seem that everything else has already been deemed inappropriate. I will qualify that with the opinion that kilts were probably not even thought of in making that determination.

If any attendee has a question about the appropriateness of their dress, please consult with Dr. Carroll or Mr. Biggs prior to the event.
This seems to me to only apply if there is a question other than does this conform to the letter of the rules. For instance a question on how low a neckline plunges or how high a skirt rises. But does not seem to apply to simple things like does this count as a collared shirt.

Violations may result in non-admittance. If dress code violations occur after admittance, student/attendee may be
removed from the dance.
This may provide a small ray of hope in that is say may and not shall. This means that staff have some leeway bout allowing students to stay. I don't know what standard if any might be applied but there is some promise there.