It must be hidden, in this case, because the Marine Corps have said that a cap and gown are appropriately worn over the uniform where the school district compels that attire. The school district's dilemma here is that permitting this graduate to differ from the rest creates a problem with other graduates who've managed equally-laudable accomplishments, wanting to display those accomplishments in some formal fashion. If you make an exception for this kid, you're hard-pressed to deny exceptions for others. Further, if you make an exception here and you deny exceptions for others who believe their accomplishments equally worthy of public display, the school district will face forms of protest and civil disobedience from those who've been denied the opportunity to dress as they wish.
It's pretty simple: there's a rule and the school district wants the rule applied strictly. This is what I don't get -- even the Marine Corps has said the School's practice is permissible and has advised the kid to just wear the cap and gown. Where's the problem?