When I finished my internship, what we learn in school, it seemed was only a fraction of what we would need when we started working. There are some things you can only learn by doing and no amount of theory can really prepare you for what is out there.
An internship bridges the gap between school and real life.
You realize the importance of things you never thought to be important before. You realize how essential team work is. How important it is to have good communication skills. How to cope with all kinds of people, deadlines, pressures and obstacles. An internship can teach you how to prioritize. You learn those skills you have only heard about vaguely like team work and dedication and which I am sure at this point doesn't sound as important as learning your trigonometry. But trust me when I say that there is a lot more you are going to have to do when it comes to actually starting out in the 'real' world.
When you go for an Internship, don't make the mistake of trying to be in your comfort zone. Now is too early for you to define yourself by only a certain kind of work. Do something you never did before. And you will learn something you never knew before. Don't make the mistake of defining yourself and keeping within the boundaries of what other people think you are. You love business? Do an internship involving research. You love design? Do an internship where you will learn about business. You have a lifetime to do what you love. Right now, at 16 or 17, you HAVE to explore.
Another thing you should know is that it might not be 'fun'. But remember that is the point. You are there to learn. And as a wise man once said, "Experience is the most brutal of all teachers. But it teaches you the most."
Don't go for the internship only to make your resume look better. Don't go because you want to make your teachers or parents happy. Go to learn something you never knew before. Get out of your comfort zone because that is the only way we grow.