Is there a reason you need to stay a resident of TN? South Dakota residence status can be had with a single weekday trip up there, and SD allows 18-20 year olds to get permits, which then would be honored in TN - since you'd be a SD resident not a TN resident.
Unless you have some need for TN residence status (break on tuition at college), this method would work and would be completely legal.
SD also doesn't have an income tax (which makes it a lot better state of residency than say GA or AL). And the process to claim residency in SD is very simple and requires very little time in the state (as little as one night).