A school's proximity to your home or work should be taken into consideration prior to signing up. Although an hour commute to class might not seem too bad at first, keep in mind that you will be making that drive two-to-three times a week for the next several years. Find a school that fits your needs, but is also within an acceptable driving distance.
Take notice of the school atmosphere, the attitudes of the students and instructors. Are they friendly and respectful toward one another? Do they appear to be having fun while free-sparring or do they show irritation and anger? Does the instructor appear to enjoy teaching? Are there an unreasonable number of injuries in class caused by a lack of control? Overall, does it seem like a place you would like to spend 3 to 4 nights a week for the next several years?
If you see students engaging in sparring and smiling and laughing at the same time, there is too much sport involved and not enough serious training. If they never smile, then they are too serious and probably not having fun. If everyone laughs and talks all the time, the school is most a social gathering, not a serious Taekwondo dojang. When someone does something dumb, such as falling while trying to kick too high, the students and instructors should laugh with, not at, the person.