This question comes up periodically, most frequently in the context of college students.
Here's what the atf says:
http://www.atf.gov/pub/fire-explo_pub/qanda.htm
(B11) What constitutes residency in a State?
The State of residence is the State in which an individual regularly resides or maintains his home. A member of the Armed Forces on active duty is a resident of the State in which his permanent duty station is located. If a member of the Armed Forces maintains his home in one State and his permanent duty station is in a nearby State to which he commutes each day, then he may purchase a firearm in either the State where he is stationed or where he maintains his home.
[18 U.S.C. 921(b) and 922(b)(3), 27 CFR 178.11]
(B12) May a person who resides in one State and owns property in another State purchase a handgun in either State?
If a person maintains a home in two States and resides in both States for certain periods of the year, he may, during the period of time he actually resides in a particular State, purchase a handgun in that State. But simply owning property in another State does not qualify the person to purchase a handgun in that State.
As you point out, you may in fact be a resident of your school's state during the period you're in school. This status may or may not be modified by the laws of your "school" state.
Invariably, FFLs balk when they see the "home" state driver's license, reasonably suspecting someone's trying to pull a fast one, which is bad mojo for them. Honestly, I'd say that the youth aspect of a college aged person is another yellow flag in the FFL's mind.
There are ways to document residence in the "school" state, (lease, rental agreement, utility bills, etc) but it seems that whether the FFL is willing to accept them is an open question.
The _only_ time I'd ever seen a similiar situation work was with a middle aged man who lived most of the year in NJ, but had summer property in PA. The FFL accepted the DEED TO THE PROPERTY as proof of at least part time residency.
At the end of the day, when it comes to irregular, possibly valid transactions, the FFL will err on the side of safety...HIS safety.
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