It's fine for high schools to require graduation gowns for all graduating seniors, including military members
An Illinois high school told Marine Corps Pvt. Megan Howerton that she had to wear the standard cap and gown for graduation, rather than the military dress uniform that Howerton wanted to wear. As a result, Howerton didn't participate in the graduation, and the high school is being faulted for its decision; see this NBC Chicago story for details. (Note that the high school had no objection to Howerton wearing the cap and gown over her uniform, but Marine protocol apparently prohibits this.)
I have great respect for our military, but I think that the high school is in the right here, as I wrote when a similar story arose in 2013.
1. First, the purpose of a graduation is to mark the departure from school, not to indicate what one will be doing after school. Some might go to the military, some might go to college, some might become baseball players. But the point of the cap and gown is to focus on what all the graduates have accomplished, not on what some might be doing next.
2. Second, and more important, allowing one person to have distinctive dress draws undue attention to that person, at the expense of the others whom she will overshadow. The uniform cap and gown highlights the uniform accomplishment that is being honored at this event. There will be plenty of time to honor the Marine's future individual accomplishments.
None of this, of course, in any way diminishes the importance of military service or the courage that it requires. It's just that this facet of Howerton's life ought to be celebrated on another occasion; this occasion is for celebrating a different facet of her life and her fellow graduates' lives.
3. Some schools may take a different view, and indeed Pennsylvania and New Hampshire have laws requiring schools to let members of the military wear their uniforms when participating in the graduation ceremony. The New Hampshire law, enacted earlier this month, was named after Marine Lance Cpl. Brandon Garabrant, who was killed in Afghanistan in 2014, and who had been barred from wearing his military uniform to his 2013 graduation. I appreciate the sentiment behind that approach, but for the reasons I gave above I think the policy requiring all graduates to wear caps and gowns is the better approach.
4. What should the rule be when it comes to religious accommodations, another matter that occasionally arises? I'm inclined to say that this matter is different for two reasons. First, religious accommodations are generally given in response to what people sincerely feel is their religious obligation, or at least what they feel strongly religiously encouraged to do. The sense is that we shouldn't require people to violate their felt duty to God in order to enjoy their graduation ceremony. While Howerton is naturally proud of being a Marine, I doubt that she feels a similarly strong sense of obligation to wear the military uniform everywhere. (Indeed, a Marine spokesman noted that Marine policy does not require the wearing of the uniform to graduations.)
Second, most religious accommodations tend to involve less obtrusive and distracting items than an entire uniform. Naturally students will not be entirely uniform—they'll have individual faces, individual hairstyles, often minor bits of individual jewelry (such as earrings). A headscarf worn under the cap may be more noticeable, but not by a great deal; a turban worn instead of the cap may be still more noticeable, but still not as much as a full-on uniform.
Now in theory there might be some requests for more obtrusive religious accommodations, such as someone asking to wear a nun's habit or a saffron monk's robe; but it's not clear to me that such accommodation requests should be granted, and if they are granted, that should only be for the first reason given above (the sense that the people feel a religious obligation to wear the garments). In any event, these theoretical obtrusive accommodations aren't what we generally think of when we envision the typical religious accommodation for headgear or jewelry. And a military uniform will stand out among the graduates considerably more than a headscarf would.
Incidentally, the military itself recognizes the importance of how obtrusive a religious deviation from the uniform rules would be—its rule as to accommodations of religious headgear and the like is that "items of religious apparel" are allowed if they are "discreet, tidy, and not dissonant or showy in style, size, design, brightness, or color."