Its very much possible that your son is not comfortable with the environment in school/class. Maybe he is not getting good response from his teacher or his fellow classmates. Please try to find out how is the school environment different and what is making him feel uncomfortable.

Maybe you need to the teacher for this.

genie,

You should not call this lack of confidence. As you have mentioned he is not shy or reserved in public apart from school. Definitely its something in the school that he is not comfortable with. Maybe the way he has to seated and act according to someone instructions.

Or maybe he has not been around with a lot of adults apart from parents. So, he is trying to act different with the teacher. Talk to the teacher and try to make the environment more friendly. May be you can ask the teacher to behave like a family member.

Don't worry you can coordinate things with the school authorities and teacher and get it right. But do it soon as you would not want your son to be someone who he is not for long.

genie, from what I can get your child is not shy by nature. Its just that some particular place he is not adjusting. As you have said he is fine in park with other kids of his age. Then why not in school. Maybe he is fine with kids around but not used to adults apart from you and his dad. So, he is not able to accept the presence of teacher.

I can tell this because of my son faced similar problems. He was not comfortable with the way his playschool teacher interacted with him so he just sat in class doing nothing. I solved the problem by discussing this issue with his teacher. The teacher started being more friendly with him rather than just plainly instructing what needs to be done and my son was fine.

So, you need to do the same try to talk to school people and make some changes in the school environment to suit your son.