Good communication is key to success in any school, including in a dance school. For this reason, evaluate how well the dance instructors are organized and how well they communicate information, movements and corrections to you. This will determine the success you see and feel.
A good dance school is a positive place to be and work in. Anyone that is going to be attending the school should be able to benefit from a positive environment, then. If you are unsure of the environment and are worried it isn't positive sit in during a session.