My son started preschool today -- he'll go 3 days a week for 2 1/2 hours. He has HUGE separation issues so my daughter and I stayed with him for most of the time. I did leave them alone for the last 15 - 20 minutes. I'm hoping tomorrow to leave for most of the time.
BTW, it is a developmental preschool through the school district. He qualifies for speech, fine motor and some social (the separation stuff). I had hoped to get my daughter in too but they are too full to take "normal" kids. It will be good for them to have the time away from each other but I know she'd love to go too.