Many kids unwind -- and unravel -- after school. Too often parents get the message from the school that they are doing something wrong, and that's why their child is having these problems. Even parents who do everything right still have kids who are difficult to manage in the relatively unstructured environment of the house and neighborhood. If your child is taking medication for this condition during the day, and it wears off by the time school is out, or by dinnertime, there's another reason for his behavior.
The school shouldn't be considering reintegrating your son into the regular (less structured) classroom unless he's ready. Ask them to convince you that he is. Have them show you behavioral records, videotapes, or the comments of other teachers who have work with your son in a "trial" inclusion program. Try to visit the school and observe your child (preferably unseen by him) to see how he behaves. How do the teachers help him or get him to behave better than he does at home?
If a visit's not feasible, then ask the school to videotape your son in several situations (small group, large group, and recess, for example) so you can compare what happens in school and at home. You can also invite the teacher or school psychologist to observe your interactions with your son at home, and give you advice. You might find it helpful to enroll in a group for parents of kids with ADHD that focuses on how to manage your kids better at home. Ask the school to work with you to help your son behave appropriately in both environments.