Jackie   Thank you for your question and welcome to ADHDCentral.com. I am not a medical professional, however, even if I were one, it would be impossible to say what happened and why your son is acting defiantly.I would start with talking with the school to find out what is different this year and whether there are any stressors that may be causing this behavior.   Is your son currently in therapy and receiving treatment for defiant behaviors, such as for Oppositional Defiance Disorder?   If you are finding treatment is just not working, you may want to schedule an appointment with a psychiatrist to make sure you have an accurate diagnosis.   For more information:   When Treatment isn't Working: The Importance of an Accurate Diagnosis   ADHD in Elementary School   Oppositional Defiant Disorder   Conduct Disorder   I hope this helps   Eileen