What are are the qualifications for the school and teacher? Are they licensed? What education and experience does the teacher have. If she is well qualified I'd make an appointment to ask her to expand on what is happening? She may be able to explain herself better in person with a dedicated block of time.
I urge you to also visit in the classroom on several different days so that you can see for yourself what is happening. Keep an open mind.
I don't know what you mean by criticizing him in front of other children. If she's correcting his behavior in a professional manner it is necessary. Your son does have to learn to conform.
When she says he's in his own world, she may be indicating a learning disability. My grandson, 7, has always been in his own world. He's been in special ed and is now in a therapeutic classroom. It's good to be unique but he also has to learn to follow the rules.
I suggest that you make an appointment with the school district for an evaluation for learning disabilities since he's having difficulties. Best to get help with them now if he has some special needs. But, also, this would help you to talk with the teacher if he tests out to not have learning disabilities.
My granddaughter, who is now 10, had difficulties focusing and staying in her seat in Kindergarten. She had an experienced and creative teacher who found ways to ground her. One thing that she did was have her sit on a chair during circle time. The other kids sat on the floor but the floor was too wide an expanse for my daughter. The chair gave her a specific place to stay.
My granddaughter has become a good student. She's in the 5th grade and had an IEP, Individual Education Plan, last year. She received some extra help with math and reading and is doing well this year. I wish we'd gotten help for her sooner. She did get help thru the school district but it took this long to know what questions to ask and who to ask them of.
Start with the school district office if the principal of the school doesn't refer you to the County Intermediate School District office. That office will perform an evaluation and make recommendations for handling your child's situation if he has a learning disability.
Considering learning disabilities is often scary but it needn't be. It could just be that your son is not ready or not mature enough to benefit from a structured classroom setting. Or he may need some specific help. My grandson became easier to be around after his mother received some help with learning a different way of parenting him that was based on his personality and needs.
I am confused about your description of him being introverted as compared to being an inborn performer. Telling stories in a TV program doesn't fit with being an introvert. For me, this indicates a possibility that he is confused about what is expected of him. Perhaps you're encouraging him to perform, to counter balance the being quiet and reflective and he's having to work at maintaining that outgoing personality. Or perhaps he's really an outgoing kid but because you're thinking of him as introverted you're not able to teach him to control his own behavior. Just an idea based on too little information.