Becuase of prior work experience in the professional dress retail sector, I continue to assist with Dress for Success seminars in the area. Working with local organizations and addressing career classes in high schools, this seems to be a very common question for young people. My advice is always the same: err on the side of conservative. The higher the position, or older you are, the attire usually changes to a dress suit. While in high school, most of your interviews may lend to the business casual attire that has already been suggested, but do your research as to what the environment of the workplace is and if a full suit is required even now, then either wear a basic colored suit or a blue blazer/dark pants combination. Shirt should always be a basic blue or white and a subtle patterned or plain tie. (For young women, simple skirt or pants/blouse or sweater or dress. Nothing overly short!) No loud jewelry or watches, hair should be simple and not styled in a distractive fashion, and dress shoes, or for young male teens, sometimes a simple brown boat shoe might work in the right enviroment and with khakis. And yes! Everything is ironed and clean! For young girls, no spiked heels. Maybe a simple pump or flats. TV may show people showing up in outlandish outfits, but in the real world, that doesn't work.
The idea is that you want to make a quick, positive impression. You never want your clothes to become the focal point and be a distraction to the interviewee. You want them to remember what you talk about, your experience, not have the lasting impression be the loud tie you wore or the fact there was too much product in your hair!!
As you get into college interviews in some schools, you will find this also important and depending on the school you may also need to research what is expected. One of my sons had to wear full suit attire for his music school interview.
Remember: simple, basic and subtle is best. And good luck!