As Dottie mentioned, with private schools it's common that kids in a given class this year (the Kindergarten, in this case) won't return next year.
I suppose it's possible for no one to leave the class, but that would be, well, odd in my experience. The school may well be coveted, but there are other reasons for leaving: families move, people lose jobs, people change their minds about their goals for their kids, etc. etc.
Take a deep breath and try to forget about it for a while (being stressed out won't do anything but make you unhappy anyway).
