You can search the myriad of threads on this topic, but I'll give you a quick run down. The definition of "domicile" (the law's word for residency) states that your domicile is changed whenever you are present in a state with the intent to remain there indefinitely. Because people have a motive for lying about this, a person's actions are examined to show there intent. In other words, what did you do to show that you intended to remain -- buy property, register your car, get a new driver's licenses, get a job....
Now states know that people will try to change their domicile in order to qualify for instate tuition. So they have enacted laws that deny new residents instate tuition. Each state set's its own rules for this. Some are quite easy, while others, like Massachusetts are downright draconian. That's the bad news. The good news is that the school itself interprets the rules and may be more genorous than technically allowed by state law. Believe it or not, it is possible to not qualify for instate tuition (or admissions preference) in any state.
For most med school applicants who have gone straight through, they will continue to be residents of the state where they graduated high school. There are many many many exceptions to this, however.
As earlierly advised, you should check with the school.
Good luck,
Ed