I am not giving you any legal advice herein, as that would violate the forum rules, I am speaking as a fellow autism parent. I am also not admitted to practice anywhere but in New York State, and I do not even pretend to know the state laws in any other state. However, I am a practicing lawyer [over 20 years experience] and a sitting judge. I am also a parent of three beautiful children, two of whom are on the autism spectrum. Naturally, I specialize in Special Education law in my law practice.

I don't need to give you any legal advice, because I think you have been given some very good advice already about how to present the situation to the school administration. You also seem to be a very good and experienced advocate in your own right, and have dealt successfully with your son's school district for many years. I think you are very much on the right track to use the strategies and skills that have produced success for you in the past in this situation as well.

As you know, the contract itself suggests with whom to discuss this matter:

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 also would suggest that there is a substantial probability that, as Forrester Modern has opined, just showing up in a kilt and hoping to blow through the front door may result in a negative outcome. This language suggests that they would prefer that you consult with them in advance about anything that potentially involves a decision about the Dress Code.

...
MALE ATTENDEES:
a. Must wear collared shirts, slacks and tie apparel.
b. No jeans or shorts are permitted.
c. Undershirts may not be worn in place of collared shirt.
d. Collared shirts must be worn throughout the dance; bare chest may not be exposed.
e. Any other clothing or dress deemed inappropriate by the Mission Viejo High School Administration.

The requirement for slacks would seem to prohibit anything but slacks. When you discuss it with them you will find out whether they will insist on the literal terms of this provision, but it could be a good idea to be prepared with a presentation about kilted formal attire and your family heritage, and I agree that explicitly promising to wear undergarments may raise the decision maker's comfort level.

As you can imagine, school districts are allowed a significant amount of lattitude in controlling student behavior for the maintenance of a safe and orderly environment, and although the doctrine has been modified, schools are still considered "in loco parentis", which in this situation means that they get to make rules about the way people dress. They seem to have done a thorough job in writing their behavior contract, and have stated their rules in as neutral a way as possible, which may increase their enforceability, as everyone gets fair, advance notice of what is allowed, and all are provided a means of discussing any potential issues in advance.

And yes, their decisions and rules may still be contested if they violate civil rights, but as I think you have indicated in several of your posts here, you are more inclined to try to persuade in the first instance. Along with my kilted brethren and sistern here on the forum, I would email, post, write letters, make telephone calls in support of any protest you need to make in the event of an adverse decision, BUT as an autism parent, I try to pick my battles, and try to avoid having to battle at all by means of successfull persuasion, where that can work.

The same kind of preparation that you have used in the past may well result in agreement, here, too.

You may be pleasantly surprized by a favorable decision. That being said, one problem you may encounter is the idea that if they let your son wear a kilt, they will have to let any other males wear a skirt or dress who choose to in the future. If you can anticipate that concern and give them a path by which to avoid that result, you may be able to overcome the objection.

Perhaps the point to make in response to this objection is that a kilt is a male garment appropriate for formal occasions, and here is where photos of kilted gentlement in formal rig, as suggested by others herein, would be quite useful, and you may consider including photos not just of Prince Charles, but also Sean Connery, whose formal kilted attire may be even more persuasive to an American audience, who think of him as the dashing James Bond. You are not opening the door to cross dressing skirt and dress wearers, you are wearing a traditional male garment consistent with standards of semi-formal events, in full compliance with their guidelines.

I also think that the suggestions previously made about considering having your own graduation event at which your son's kilt can be featured has a lot of merit.

It seems from your previous posts that you have already discussed the potential controversy and possible denial of permission with your son. I would continue to discuss the possibility of denial with him, and include him in the planning of what to do if permission is denied.

Having your own event at which he can be kilted could be a good way of having your cake and eating it, too, without feeling like you have to start a litigation war.

Another possible alternative solution could be to rent tartan trews to wear at the prom, and go kilted to your own private event. Trews would seem to comply with their Dress Code without question, and allow the celebration of your Scottish heritage.

Whatever options you consider for the eventuality that permission is denied, as I am sure I do not need to tell you, if you have prepared your son, he will be better able to absorb the denial and still enjoy the event without perseverating so much about the denial that he cannot enjoy the event.

Good luck to you and your son. It does not surprize me in the least that you would take a class to learn how to make a kilt for your son. That is just one item in a life long list of things you have done for him over many years to improve his lot in life.

Not many children on the autism spectrum get to go to a High School prom with their typical peers. Your son obviously is headed toward a wonderful outcome. That does not happen by accident. That happened here because you are a ferocious advocate, and because you have dedicated yourself heart, body and soul to your son's development, scraping out the development of skills inch by inch, day by day, year after year after year. That happened because you are a rare and beautiful person who gave your son the path to life itself by putting his needs above yours every waking moment. That is the true meaning of love.

God bless you.