If you saw one of your Joes doing this, how would you react?

Well first, I would tell him to take it down. Inform him that he is a reflection of his branch and if he makes a fool out of himself, then he makes a fool of the whole branch. Explain what he did wrong. He might genuinely not know. I know Marines can't wear cammies off bases, but since other services can, he might have thought there was no problem. I also know most other branches have some form of a dress uniform which would have been more appropriate. Then I would go into the other dangers... if he was over age and his date was underage. If there was underage liquor at a prom party (because we all know what happens at proms). Make sure he understands that as a service member, he is held to a higher standard than all the friends he used to hang out with.

I wouldn't do paperwork on him as long as this was kept in house. If someone higher on the chain brought this to your attention, your hands might be tied. But you need to bring it to his attention that if you can see the picture, so can others and he should remove it before he's caught again.

Being a newbie and making a boneheaded mistake while trying to be cool back home is something that needs to be brought to his attention.

But laying the bonehead out on the internet for all to see (even though we don't know who this is) is still potentially humiliating for this soldier if he were to also be on RP. It doesn't require a meeting of the minds to remind him he was not only out of regs, but also a total douche. Be a good battle buddy and don't blow little things like this up.

As it was stated above, we are all just speculating. But I work in a high school and this is definitely home coming. The cheerleaders were awarded their sashes, because they and the boys were probably part of the homecoming court. That young soldier was around and decided to take the picture with the homecoming court. No big deal. I guess the person that wants to reprimand the young soldier never went to prom. Cheerleaders don't wear their uniforms to prom.

I'm more concerned about why you are asking this in a public forum rather than addressing it with your supervisor? Yes I know, this forum is here to help at times. However, it does not negate the fact that your immediate supervisor is there to assist and mentor you as part of his job.
I find this whole conversation idiotic, and think we have even more important issues we could waste time on rather than a soldier wearing ACUs to a high school function. No matter if I do think he looks idiotic, there's still not enough info to respond with certainty anyhow.

I am a military historian afflicted with a Cavalry smart mouth so I have met some very interesting characters over the years one was Ian V. Hogg (1926 – 7 March 2002) who was a real piece enlisted in the Royal Artillery of the British Army in April 1945. During World War II he served in Europe and in eastern Asia. After the war he remained in the military. In the early 1950s, he served in the Korean War. Altogether he served in the military for 27 years. Upon retiring in 1972, he held the appointment of Master Gunner at the Royal Military College of Science, where he taught on the subjects of firearms, artillery, and their ammunition and use. Hogg has been described by publishing people who worked with him as "an unassuming man, with a gift to pass on his knowledge at any level, and often with a dry humor" That comment about “dry humor” is typical British understatement Master Gunner Hogg told a story which to this day I do not if it was true or made up out of whole cloth but it brought down the house with laughter. It went like this “During the Second World War a subtenant of the Royal Horse Artillery was seen chasing a beautiful nude young lady through the hallways of Shepard’s Hotel in Cairo and he was in the same condition he was charged with “conduct unbecoming a gentleman holding the King’s commission”. During the courts martial the learned young barrister who was the council for the defense pointed that King’s Regulations stated that when not in uniform members of the service should be properly attired for the activates they were participating in. The courts martial agreed and he was acquitted.

So let's turn a negative into a positive. He was so proud of that uniform that he wore it to his prom.

k. Wearing Army uniforms is prohibited in the following situations: (1) In connection with the furtherance of any political or commercial interests, or when engaged in off-duty civilian employment.
(2) When participating in public speeches, interviews, picket lines, marches, rallies, or public demonstrations, except as authorized by the first O–5 in the chain of command.
(3) When attending any meeting or event that is a function of, or is sponsored by, an extremist organization. (4) When wearing the uniform would bring discredit upon the Army, as determined by the Commander.
(5) When specifically prohibited by Army regulations.
l. Soldiers will wear headgear with the Army uniform, except under the following circumstances:

So he may be a dork fish by doing so. But he didn't violate Ar 670-1

Instead of Prom, could it have been Homecoming? Cheerleaders wouldn't wear uniforms to Prom.

I had a soldier that was already Married and went to prom their is nothing wrong with it. Kids can join at 17. Now as far as him wearing ACU's that's kinda half ass and it looks like a homecoming dance not prom.

This doesn't look like a prom to me, but more like homecoming so way less formal. I was in the Navy, but my husband is Army and we live on post in an Army town. I see soldiers in this uniform everywhere so this is not surprising.