“We sent our son to the JCC for nursery and pre-K. It was basically like a public school with many who were not Jewish or religious. He was the only religious kid in his class there. He felt quite uncomfortable and out-of-place, and he did not do well. At 5 he had the personal and social development of a 3 year old. He was very insecure and mistrusting.”
this is not typical though. your child was unhappy, and for whatever reason the way it manifested itself was in stunted social development. a typical child will do just fine in a public school setting.