What are your basic requirements for hiring? I mean is a Degree the only qualification they need to have or do you ask them to be TESL/TEFL Certified as well. People who have taken the time and effort to get qualified in the field are likely to be more serious about teaching. Also have you considered asking your short listed candidates to give you a sample lesson with one of your classes before making them a firm offer? If you recruit your teachers from abroad that could be not feasible but you could always include in your telephone interview a few questions aimed at checking their understanding of teaching techniques and their resourcefulness when it comes to classroom management. You could set a couple of scenarios and see how they respond to it.
Hope this helps