While in undergrad ROTC, you have no worries about deployment (wasn't sure if that was part of your original question or if you were just stating it).
Once you are graduated, it is entirely up to the Army whether or not you will be allowed to go to medical school if you are accepted. To the best of my knowledge, there is NO way anyone can guaruntee it to you at this stage.
In the past, many people who graduated & received commisions from ROTC have been allowed to attend medical school if accepted. There is NO guaruntee that this policy will continue in the future.
Worst case scenario: you graduate ROTC, get comissioned, are not allowed to attend medical school, and serve as a whatever for 4-8 years. The possibility does exist to attend medical school before your AD comittment is finished, but again, there are never any guaruntees.
I just re-read your post and realized you will be going to the guard for your commitment - Since you will be serving in the Guard, you might have more leeway, since the NG tends to be more insular and people tend to know each other (ie, after a year or so your battalion commander goes to bat for you to get released for medical school) Also, if you're still in the guard, there's nothing that says your "day job" can't be medical student, always realizing that medical school (particularly the last 2 years) is not exactly a 9-5 job, and occasionally will conflict with required drills.
My personal experience with the guard (3 years up to & including 9-11) has been that they are very understanding as far as people missing drills for valid reasons, but if your unit becomes activiated to deploy to a real-world mission, that of course changes.
Hope this helps,