I'm sorry to hear about your difficulties. How old is your son/what grade? To answer your first question, ALL children have behavior problems at one time or another, but it's the degree and frequency of their behaviors that makes it an issue, compared to their peers. Two-year-olds are expected to have tantrums at times, but the same behaviors in a ten-year-old are cause for concern. All kids have "off days" once in awhile, but the description you give of screaming at the teacher does sound like it was more than that.

When was your son diagnosed with ADHD, and how are you treating it? Is he taking any medication?
Has your son had other times where he gets very angry with the teacher, or other adults? There are other conditions that are often seen with ADHD... one is ODD. (oppositional defiant disorder)

Here is one description:
"Oppositional defiant disorder (ODD) is a behavior disorder, usually diagnosed in childhood, that is characterized by uncooperative, defiant, negativistic, irritable, and annoying behaviors toward parents, peers, teachers, and other authority figures. Children and adolescents with ODD are more distressing or troubling to others than they are distressed or troubled themselves.

Most symptoms seen in children and adolescents with oppositional defiant disorder also occur at times in children without this disorder, especially around the ages or 2 or 3, or during the teenage years. Many children, especially when they are tired, hungry, or upset, tend to disobey, argue with parents, or defy authority. However, in children and adolescents with oppositional defiant disorder, these symptoms occur more frequently and interfere with learning, school adjustment, and, sometimes, with the adolescent's relationships with others.

Symptoms of oppositional defiant disorder may include:
frequent temper tantrums
excessive arguments with adults
refusal to comply with adult requests
always questioning rules; refusal to follow rules
behavior intended to annoy or upset others, including adults
blaming others for his/her misbehaviors or mistakes
easily annoyed by others
frequently has an angry attitude
speaking harshly, or unkindly
seeking revenge
The symptoms of ODD may resemble other medical conditions or behavior problems. Always consult your adolescent's physician for a diagnosis."

It is my understanding that virtually all children who are diagnosed with ODD also have symptoms of ADHD, but not all children diagnosed with ADHD have ODD. (Similar to the statement all collies are dogs, but not all dogs are collies.) Children with ODD seem to have more outbursts, and seem more "strong willed." I would ask your pediatrician for a referral to a mental health specialist, such as child psychiatrist, who is familiar with coexisting conditions should make the diagnosis, because there are other mental health issues that could also come into play.

Here are some other sites that explain it:
www.help4adhd.org/en/treatment/coexisting/WWKB
www.webmd.com/mental-health/oppositional-defiant-disorder
www.oppositionaldefiantdisorder.ws/

I'd also like to invite you to join us in the Learning and Attention Difficulties Group at community.greatschools.net/groups/11554