The term I would use is business casual- Not dress, but not jeans either.
Neat Khakis with a presentable collared shirt depending on weather. Well groomed. IE: Don't have to have short hair, but it should be combed, shaved, etc. Until you have seen kids in PJ's & bedroom slippers at an event you won't understand. Same for boys in cut up Tshirts, flipflops and bed head.
While you may not see anyone but staffers, you may also see:
- Field force (very likely)
- Admissions reps (happened several times for my son)
- Key MOC staffers (nearly always in our area)
In all of these it's better to make a good impression. Not dressy, but neat and well groomed. which you could be in casual clothes even. Grooming probably counts more than clothes.
I would not underestimate the opportunity that can present itself in the academy days and admissions events. It's a chance to get a read on your background, associate a face/person with a name, and ideally, get an advocate batting for you. (Field force or admissions).
Same even for college fairs, though USMA did not show up at many. USNA & USCGA do in our area, and DS was put into NASS based on interaction with USNA rep at a college fair where he showed up in business casual with a resume, etc.
We believe connections made with field force and admissions at some of the USMA events helped DS with USMA as well.
I will add to the earlier comments, unless traveling as part of a jrotc group on a formal trip, the jrotc uniforms did not seem to go over as well. You can see some negative reactions. But much of that depends on the kid as well. Some jrotc applicants have an entitlement vibe which get's some negative reaction. So you can sidestep that by making sure you do not assume it gives you an edge, just make it part of your balanced package. (academics, leadership, physical, etc)