It's more than free speech, it's a national necessity. If we're not going to allow the military a draft, they have to get candidates somewhere. And just like many schools allow corporate interests on campus during job fairs and college recruiters throughout the year, there's no reason the military can't set up its booth right alongside them. Let them make their pitch, and let the individual decide for themselves, you know, like you do in any other free speech situation.
Military recruiting in schools is consistent with free speech. They are there to explain their views and why they think it would be a good idea to join. But just as they can explain their beliefs, you have the right to listen or not listen. They aren't forcing people to join, just talking about it.