F.:
If you want, let us know the name of the school. You may get some specific responses from mothers who have had direct experience with it, which would be invaluable. Staff turnover is typically not a great sign but of course there are exceptions. What type of "incidents" occurred with other children?
How did you son react to the school? Did you stick around to observe? Did you drop in at odd times to check out the atmosphere? Was the environment one where the children are required to respect each other? Was it an active preschool or more of a custodial (day care) situation? Did your son seem comfortable, did he want to go there? What was the teacher/student ratio?
Frankly, I would pay attention to your feelings. I ignored mine once...I figured just like you that "well, I guess this is just how it is" or that I was merely overprotective or that I just had the paranoia of a newbie mom. I was wrong. While I do believe that a parent is never entirely comfortable when a child is out of their midst, there is an enormous difference between finding a suitable and desireable preschool/school environment and a place that just hopefully won't let him fall into harm's way. If your preschool doesn't fall into the first category, then the preschool (at the very least) is probably not doing him any good.
We just moved to the area so I went through the whole investigative process again (yikes). One thing I have found useful is to closely observe the other children in the room or rooms. There is nothing like seeing a school's philosophy in action.
I'm sorry for the long post. I guess I'm trying to say that research and visiting lots of schools and observing and even trying them out for a day or so (the summer sessions are great for this) can be one way for you to become more comfortable with your decision! You may find that the one you are in is great and be relieved OR you might find a new one that you really do feel GOOD about entrusting your son to.