You should also take note of the aircraft that a school uses for training. The planes do not necessarily have to be brand new but they need to be properly maintained. While you might not be in a position to really assess a flight school, you could do yourself a great favor by confirming that the school you choose at least has the approval of the FAA and that the syllabus it uses is approved.
The success you are likely you are likely to enjoy at flight school will be influenced to a considerable extent by the attention you get from your instructors and you cannot hope to get adequate attention when you enroll in one of those crowded schools. Ideally, the instructor-student ratio should not exceed 1-2 and if your flying school has a ratio that is too far removed from this, you should consider looking elsewhere.