I have a 6 yr old son whom I had the same problems with in Kindergarten last year. His teacher was the same way and I took it offensively. I even reported her to the principal because I thought all she wanted to do was put my son on meds because she didn't want to deal with him. He was soing the same as your son, not concentrating and acting out, disrupting the class, not staying in his seat, talking loudly. I refused to listen to the teacher because of the way she was towards us.
Now he is in 1st grade and his new teacher said he is a very bright young man who may need to be tested for ADHD because he is showing the signs of it. She was filling out a report for another child and my son was fitting in the categories. I trust this teacher because she was my other son's teacher too and she is a wonderful teacher.
I had my son evaluated and he is now on Ritalin ER 20mg of a morning before school, Ritalin 10mg at noon and Risperadol 0.5 at night so he can rest and he is a totally different child. The meds really helped him. The teacher started doing behavior charts daily and he has been bringing home great ones for a month. He is also reading at a 4th grade level and above all the other children in his class.
This may or may not help but I felt I didn't want to put my son on meds but I am so glad I did. He used to have bad temper tantrums and shake real bad but now he don't. The meds work. Just takes time to get the right meds for your son. Hope all goes well for you.