What is the system for voting when away in college?

Absentee Ballot

Generally speaking, you can either register and vote where you are attending classes or in your home town, as long as you don't do both. In most states, you can legally vote at an address if you have lived there for at least 30 days before the election and, obviously, aren't voting elsewhere. It's usually better to vote at the address you will actually be living at on election day, since it will be easier to vote there. Some people go for strategic voting - if there is a key race that is likely to be close in one location, but not at the other, they register where they expect the vote to have most impact.

Election laws vary from state to state.

In Tennessee, you may register in your hometown or in your college town. If you register in your hometown you will probably need to vote by absentee ballot. To do this you will have had to register "in-person" and submit (by letter or fax) a written request asking for a absentee ballot be mailed to you.

Safest is to register in your home town and vote by absentee. Voter registration is one of the tests for residency which vary state by state. You wouldn't want to accidentally mess up your status.

Register in your hometown, then get an absentee ballot. Check the deadline, since you have to have the absentee ballot in before election day.