just watching Flight School Update trailer:
I can believe, the army outfits and other stuff that's went cut probably a little teaser for an upcoming DLC, which might focus on a military theme,
and i believe many of you already know the Hydra was already included but didn't made it for the Flight School, probably Rockstar thought's this might
not work well with the flight event day, so that's why there is no weapons nor armed planes/vheicles made into the the updates.
also, Flight School aimed to improve players Flying Skills, this might seem to prepare them for the next updates challanegs, which might include armed airplanes and other weapons.