You should always question any diagnosis that contradicts your gut. You can continue to pursue a second medical opinion and time as well may clarify the issues.
However, I also want to point out that the working memory score does not preclude ADHD. While it is true that one typcial profile for ADHD is low working memory, it is not a prerequisite. My DS who was diagnosed with ADHD also had a similar working memory score. Another point to consider is that if the recommended treatments help the behavior, then it may not matter even if your DS does not really have ADHD.
If the impulsive/inattentive behaviors are never observed anywhere else but at school, then I am inclined to question the diagnosis. However, keep in mind that you may have created a home environment to cater to his deficiencies and that your tolerance level is far higher than a non-family member. For example, it is easy for ADHD children to focus on what they want to do, be it legos, puzzles, computer, fun books, etc. The test is paying attention appropriate to an environment when you don't want to do so.
