The flight instructor obviously is a key element in your flight training program, which is why it is important to select a good school with a staff of instructors.
The fact is that most full-time instructors, especially at larger flight schools, have their sights set on a professional piloting career. At some point they likely will move on, leaving current students in min-training. If your instructor departs for greener career pastures, or you must change instructors for some other reason, having several other competent instructors on staff from which to choose makes the transition easier and causing less of an interruption in your training progress.
It's important to know the school's philosophy concerning its flight instructors. Is there some degree of standardization among the instructors, so that switching to another instructor won't significantly affect your training progress? Does the school have a policy of bracketing each flight lesson with pre- and post-flight briefings between student and instructor? Does the school enforce punctuality and preparedness so that instructors consistently report for lessons on time, ready to teach? (The instructor should expect the same of you.) Ask other students about this important issue.
Will you train to a syllabus? This is important because a syllabus serves as a guide for your training, and a way to track and measure your progress. Plus, if you change instructors for any reason, the new instructor can see where you're at in the syllabus and pick right up where the training left off.
Does the school support its instructors with adequate compensation and equipment? This may be difficult to determine, but you can get a good feel for instructor morale by talking to several instructors-and students-privately.
Even though you'd prefer to train with one good instructor, it can be helpful to periodically fly with another instructor who can objectively evaluate your progress. Some schools incorporate these evaluation flights, called phase checks, into the instructional strategy. Ask about it.
Some larger schools employ designated examiners who can administer check rides and issue pilot certificates to the school's students. This can be a real convenience. In some areas, examiners require several weeks lead time to schedule a check ride.