Personally, If you don't mind my saying so, I think it is the incompetency, on the part of the teacher. I have found that most teachers, whom were raised in strict households and/or were only children, they lose thier minds when a child acts out a little bit.
I have two nephews, whom were diagnosed with A.D.H.D disorders. They had teachers whom were impatient, incompetent and basically being a witch. She was thrown in a crowded room with thirty screaming children and became overwhelmed, and wanted to sedate the kids.
These two nephews were screened, in upwards of nine times each, by the family doctor and the psychologist. Both stated that there were no reasons to increase medications, but yet the teacher kept screaming. Finally, I suppose the teacher convienced the school nurse to give the kids more meds. By the time the kids came home, they were no more than "Shells" of children.
After a formal complaint was made to the School Board and we had all of the kid's doctors show up, they moved the kids to different rooms, with a teacher whom had her own kids. She was patient with the boys, and we had no more problems.
I heard later that, the same teacher, whom had made such a fuss about my nephews, was up to her neck in kids. She began her usual claims of unruly kids needing medication. this lady went so far as to call Child Services, against the parents, claiming they were neglecting the children. Rumor has it, that she was finally terminated.