I thought the socialization would be good for him, but now I'm not sure. Dear Sue, If your son is only having a problem when they leave the room then it probably is not that he is not ready for preschool. They could try to give him warnings when they are planning to leave. Also, they could explain to him that they will be back when the other teacher is finished reading the book. They probably need to tell him where they are going and what they are doing.
Some children feel better when they know more.
They need to help him adjust - this should not be a problem for the teachers to do. They probably won't need to do this for the rest of the school year. Once he gets used to the routine, he will probably stop the crying. I did get a lot out of it and my daughter has truly benefited by it. My 3 year old girl was adopted at 17 months of age from China.