When choosing a martial arts school you need to remember that you are not really choosing a martial arts school, you're choosing a martial arts instructor. 

First of all you need to know what your goals are or what are the goals that you want for your child. Is it to get a black belt, to increase discipline, self control, self confidence? All of these are great goals. So you will enter martial arts school with the idea you're going to train for three to five years to earn your black belt. Secondly you will want to sit and watch the classes. You want to see how the instructor interacts with the students. Are they a good teacher? Are they personable? Every martial arts school is different. So the style is not as important. The only style that matters really should be the teaching style of the instructor. You will most likely want an instructor who's personable, explains things well, paces the class, has good classroom management, the classes are safe, professional. It is also a plus if they are a member of the Martial Arts Teachers Association, because that indicates that the instructor is staying abreast of current trends in the industry. He's studying to become a better instructor.

Then you will want to watch the advance students. Are they impressive. Are they in shape. Are they respectful. Are they disciplined. Do they line up. You should be more interested in what the instructor can do for you or your child than in how many trophies he has. And you will get an idea of that by talking to the studens and parents and visiting the school. You should not choose a school simply because it is close or because it is cheaper, in fact there is probably a reason that it is cheaper. When you are chosing an istructor you want the most qualified instructor you can find.