Did you see the actual presentation? or just hear about it secondhand?
I used to teach in a very liberal city school system, and the military was basically a non-entity for us at the middle school. One teacher's son joined the military, and he came and did a presentation about his work in her social studies class, and it was OK with me since the recruiters were not present. Lots of our school systems in my state have JROTC programs in their high schools.
I definitely understand your concern, I share many of your same worries, but I also know that for some of the students I worked with, the military was a great option for them, and one I would want them to be exposed to--since their other career plan was "be a professional basketball/football player" .
But, if you didn't see the presentation, i think you will have a harder time convincing the school board to not allow it again. What does your principal think now that the event is over? Would she allow them in again? (my former principal was former military himself, and he was very reluctant to bring the military in because he also had some of the same mixed feelings I did).