The most important factor in choosing an ESL school are the teachers that will be teaching you. There are two kinds of ESL teachers, those who speak English as a first language and those who do not. It is preferable to find a native speaker for younger students, so they can hear then native accent and pronunciation of words. Before enrolling, try to meet the teacher, to see if he is a person you want to learn from.
Many schools offer extracurricular activities at night or on weekends. See if the activities are things you like to do and whether they are free or not, then compare with the other schools. Often they will include city tours, outdoor and sporting events, as well as nightlife activities, like going to clubs and bars. These are an excellent way to get to know the students in your class, in a social environment. This is important if you are going alone to a city where you don't know anyone. You will have a much more memorable experience and meet a lot more locals if you have somebody to do it with.
The other students in the class are an important factor because you will be spending a lot of time with them. Try to find out where most of the students are from. Studying with students from many different countries, or at least ones not predominately from yours, is a good idea. If you are from Japan and trying to study English in a class full of Japanese students, there is a good chance that you will fall back on Japanese and not learn English as well. A mixed class will ensure English being spoken. A mixed class will be more interesting because you will be around people from cultures you may know little about and finding out about them will force you to speak English.
You are going to invest your money in the school, so look in to it a little. Check out online reviews of schools on third-party sites. Also,if you are already in the city, stop by prospective schools and talk to students attending classes and listen to what they say. If most enjoy studying there, there is a good chance the school is doing something right. Talk to some teachers too; if they enjoy teaching there, it will probably reflect in their teaching, which makes for better classes. If still unsure, ask if it is possible to sit in on a class. Another important factor is number of students attending the school. If you are interested in social activities, you may want a school with many students.