Sine the main goal inevitably has to be doing well in the air force, I'm not sure that bulking vs cutting is the right question, although it would play some part. On the one hand, the lighter you are, the easier it is for your comrades to carry you if necessary and the less daily rations you'll need (although without any rations, the bigger guy will survive longer, as he's got more body weight to catabolise). On the other hand, increased muscle mass is definitely beneficial for increasing strength, and increased strength pretty much increases your capacity for other key areas of fitness (it doesn't increase all other areas in and of itself, it just gives greater potential for speed and muscular endurance).
Realistically, you're only looking at a couple months worth of training ahead of you before you turn 18. That's not a long time, so unless you're pretty close to the best shape you've ever been in, you probably won't get there in the given time frame. However, you've (hopefully) got many years ahead of you, and really if you're in the best shape of your whole life (past, present and future) at 18, then technically it's all downhill from there. So I'd recommend that you focus on building strength and conditioning in order to be prepared to safely enter basic training. For strength, do full body training (eg bench press, deadlifts, squats, overhead press, pull ups, rows) in a fairly standard 3x10 sort of format, and also train for activities that you'll need to do on campus. I'm not sure what specific fitness you'll need, but I'm guessing a lot of body weight stuff, carrying things (eg farmer's walks, waiter walks), dragging things around (eg sled drags) and pushing things around (eg prowler runs, tyre flips). Do that 3 days a week, and on another 3 days a week, do plain old cardio in very conventional forms: jog around the neighborhood, go for a swim etc. Don't beat yourself up with cardio. Start light and easy, and build up your endurance. You'll get beaten plenty at basic.
