I suspect ROTC is so well known (if not as "Reserve Officers Training Corps," at least as "the high school army class thing" and has been around for many decades that it doesn't need to be spelled out, and they might not ever think to spell it out ("ROTC" might seem like a single word like "Glee"). When you said "I wondered if that was done on purpose" I was thinking if you wondered if the school ADMINISTRATION had made them use the initials only, rather than the group members themselves trying to be uncontroversial. I've heard of gay-straight alliances in high schools (they didn't have them when I went to high school or college), but if I didn't see the full name I probably wouldn't connect GSA to it and wouldn't know what GSA is. General Student Association?
Such organizations often have banners that people can carry in parades and things, and they would be holding it, or having it behind and above their heads, in a yearbook picture. Perhaps they didn't have a banner, perhaps they didn't think to get it out for the picture, perhaps they weren't allowed...
