He has been having crying spells at school. He wants to go to school, but minor events set him off. He seems to have already developed a perfectionist schema. He cried because he was tested and didn't know how he would fare, and it turns out he was at the top of the class.
He is not pressured at home, and his father and I aren't perfectionists.
If school staff members did not know your daughter at this stage, help them to understand that he will most likely become more comfortable in groups, just as she is now. Talk with his teacher to make sure that he or she will be understanding of his tears.
Ask the teacher to send you a daily note (could be as simple as a "smiley" face or checkmark) to let you know how his day was. On days when there were no tears, he could earn special time with you or another treat.
Talk with the school counselor. He or she may be able to give your son some individual time or include him in a small group. The school counselor may also be able to give him some positive support during the school day.