One other thing I’ll add, as another example of something that steers me towards “military school,” is that the black vertical stripe along the pant legs is common among military school uniforms, particularly when the pants are otherwise gray. But then of course many such uniforms have their origin in earlier US military uniforms. It’s just a stripe. Same thing with the plume on the kepi-style cap. Not knowing the school (if it is a school) it’s hard to date it because many of the older schools dabbled in kepis for a time, perhaps following the trend of the US military proper in the latter half of the 19th century, but then switched back to the older shako-style headgear (and why not? you can stick a bigger plume on a shako and not have it look out of proportion to the cap!). Pennsylvania Military Academy, for instance, seems to have dabbled in kepis at some point, with and without the plume, but again probably many schools wore similar headgear at the time. And obviously the uniform is very different from the PMA uniforms shown there (but uniforms, too, sometimes did a back and forth, and at my own school I’ll note there was a period of time late in the civil war and its immediate aftermath when the uniforms got unusually plain, dropping down from three rows of shiny brass buttons to one or two–or no shiny at all!).
And then some military schools have always had plainer/less outlandish uniforms. And the Texas A&M Corps of Cadets has, for instance, always looked absolutely ridiculous and should be ashamed to call themselves a Corps of Cadets.
None of which answers your question. But may at least give you some closure, if closure can be had from learning that the answers you seek may be unknowable absent a clearer photograph or some biographical details which wouldn’t be fair to ask for on this forum (particularly since you’re saying it’s from another family’s collection). To be clear, I’m not asking for more details. Just saying there doesn’t appear to be a smoking gun with what’s available.