I pulled my DS out of PS K and put him in a very rigorous private school. In retrospect, he tells me that he was very anxious and distressed when he went to the new school because he believed that the academic standards were very high there and that he would not be able to compete with the kids there who had been in pre-K and K in that school. They were writing essays, reading literature at the 4th grade level, doing history timelines and computer programming etc etc. But, what helped him was a little time - in 2 months, he figured out that he was actually able to exceed most kids in his class and he also made some good friends during PE and recess. Talk to the teacher and let her know that he is not understanding class rules correctly and that he is not sure what the expectations are (being called by name vs the whole group being called) etc.
I believe that the screaming in the car is just because he is already stressed a lot and the "punishment" on top of it was too much to handle at that time for him. Kids who are 5 or 6 can have a lot of anxiety in a new situation. Arrange for a teacher conference and request the teacher to remind your son gently if he is not following along. I believe that he will be fine in a few weeks. Good luck.
