I started off at McDonalds at 15, and can say it was a wise decision. I've worked many fast food places throughout the years, and now, when times are rough, I can simply walk into a fast food restraunt, put down my resume, and be almost a guarantee hire.
You said they don't teach you anything, that is wrong. They teach you the basics of employment. You learn how to work with a team, how to develop good customer service skills, how to properly follow code, how to maintain a good cash policy, and you also learn what you want in a leader, which in turn makes you a better leader.
So many people dismiss the fast food restaurants as careers as they see themselves doing the same thing 30 years down the road. Lifers, however, are few and far between. You're simply looking for easy cash, and a place to start off. It will teach you to work in a high pace environment, which is necessary in all walks of life.
While I don't recommend spending more than a few months, or years, in each restaurant, and you surely shouldn't stop going to school and learning new talents, I can think of no better place for a young adult to start their career.