If you get a rejection, it maybe hard to reverse the decision. However, some schools may look at your file again (chances being slim). I personally don't know of anyone that did, but people here may know.
All the schools that I have applied to state very clearly that once a rejection decision is made, that decision is final for that application cycle. You will have to reapply again the next year. A letter of intent is only useful when you are on the waiting list.
At this point, the best thing to do is meet with an admissions advisor from the school and discuss how to improve your app. If that school is your dream school, it would be appropriate to let the advisor know, and then to work your hardest to follow their advice this year. Schools are often sympathetic to reapplicants who improve their app and have shown a great interest in their school. Good luck, applying to med school is rough.