That all depends on how mature and reliable your intern is. As easy as it is to use, Twitter can be a dangerous weapon: it's very tempting, on the spur of the moment, to send out a 140-character tweet commenting on a news event, and then realize that you should have weighed your words more carefully. Practically every week, a celebrity or politician lands himself in hot water by not thinking before tweeting—at which point disgruntled and disappointed fans react by posting their own tweets lambasting the original tweeter, and the whole thing lands on the front page of CNN or MSNBC.
If you're going to let your college intern handle your tweets, you should give her some strict rules about what is and is not admissible. You want the tweets coming from your law firm to be professional, useful and reasonably restrained. If there's been a horrible car crash in your area, it's better to tweet something like “we send our condolences to the victims of the terrible crash on I-90...as car accident lawyers, we know how painful a time this can be” than “Gruesome crash on I-90! Click here for the latest pix!”
Remember, the point of your tweets is to tastefully call attention to your firm, boost your Internet visibility, and (hopefully) attract prospective clients, not to show off how hip or clever you are. If you don't feel that your intern can handle the task responsibly, it will only take you a few minutes a day to do it yourself.