My 11-year-old sixth-grader, who has ADHD, was in the gifted program. He has an IEP and a 504 plan. He's now in an alternative-education school for the remainder of this school year. We'll be having an IEP review before he returns. I've heard a lot about special education, but I'm not sure if he qualifies. He's had a lot of difficulty in school as a direct result of his ADHD: poor concentration, impulsiveness, bad judgment calls, and immaturity.
In general, children attend special-education classes if their academic and/or social needs cannot be met in a less restrictive environment (e.g., in a regular class with additional support either in or outside of the classroom). Special-education classes are usually smaller and teachers are trained to meet the needs of a broader spectrum of children.
You don't say whether your son is on medication for his ADHD. If he is and he's still having such difficulties with concentration, impulsiveness, etc., it sounds like the medication may not be monitored adequately.
Has any direct intervention been prescribed for his difficulties in class? For example, social-skills training classes or a behavior-modification program with very specific goals and charting of progress? These interventions do not have to take place within a special-education class -- but they certainly can.