How exciting! I really want to see you stick this through. Remember, the first draft doesn't have to be perfect...it just has to have a beginning, a middle and an end! Especially an end.
Your target length is pretty short...but I have that problem, too, sometimes. The first draft of the first novel I finished through this thread was about 30k shorter than it needed to be. Then I got some fantastic advice from another writer here at AW, put in a new subplot and made the antagonist a POV character, and got about 20k more words. It still got agent rejections (those who made personal comments mentioned the length), but it's currently being made into an e-book with a very reputable publisher.
My suggestion is to go ahead and shoot for 40k. Get that taste of sweet victory when you finally hit "The End". Then go back, add some subplots, play around with POV, and see if you can't add at least 10 or 20k. That would still put you a little short, but perhaps in the publishable range for YA.
Whatever you decide, good luck! And see this one through to the end!