I need a lot more information to give you a really meaningful answer. For example, is Zachary an only child? Is this his first experience with a large group of other children? How many hours a day does he attend? Were you able to stay with him for the first several days he attended? Does he resist being left? Is he happy when you pick him up? What kinds of separation experiences has he had earlier?
Answers to all these questions would help pinpoint Zachary’s own personal reaction. Many children are shy and cautious when they first enter a group of rambunctious kids, and it takes them a while to warm up and participate fully. This could be his situation, in which case he will soon become more involved without any special effort on anyone’s part. Or he may be trying to cope with separation fears. It is important that you make a special effort to let him know when you will be there to pick him up and that you be there when you said you would (“right after nap,” “when the children go outside,” etc.).
And I would make an appointment with Zachary’s teacher. Discuss your concerns with her and find out more about his behavior when you are not there. Finally, without labeling your concerns, talk to Zachary about it. Every school night, just before bedtime, ask him to tell you something good that happened at school that day. (You may need to relate something good that happened to you to help him get started.)
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.