Gosh, I think it's time to email the teacher, principal and counselor and say something like,

"I've been trying to figure out what's going on in class, as my son is coming home extremely upset and now no longer wants to go to school in the mornings. This is out of character for him, as he is an eager, quick learner. I would like to observe class for a day, but have not been given permission.

I would like to meet ASAP to help solve this problem. But in the meantime, I will be keeping him home from school until we can figure out a resolution that involves me understanding and observing the classroom dynamics so that I can better help my son."

Personally, I wouldn't let my child attend a school where I wasn't welcome at any time of the day.