As a prior 15E WSO, I have experience in this area. The first thing I did was call my detailer in San Antonio and receive permission to be released if I was accepted into medical school. I did, however, need my supervisor to call and gently remind them of my situation, which was I could separate prior to taking another assignment in the Strike Eagle. If they didn't give me the "permission to apply" letter, I would end up going to med school as a civilian. If they did give me the letter, I would still serve in the AF. So, I was in the driver's seat. But either way, I wasn't going back to the jet and my supervisor worked w/ the detailer to help them come to that realization. I did, however, have a year of flying commitment left for my instructor upgrade which they tacked onto my payback as a doctor. But it worked out in the end.
Where are you with your commitment? If you have the option to go civilian, I'd take it. I interviewed at USUHS and didn't get accepted. But I'm glad I'm at a civilian school because I seriously doubt I'd be able to support my family of 5 on 2LT pay. I have a much better life as a civilian than I would at USUHS. Plus, the big doctor bonuses come when you sign multi-year contracts w/ the government after your commitment is up. That 7 year obligation from USUHS will put off the big bucks for a while.
In conclusion, I would seriously consider other options available to you if you can't get the permission letter. I wear scrubs everyday and had a goaT for a while. A four year civilian tour in the middle of a military career is actually pretty fun...! PM me if you want more info.