I have seen from my experience that these two diagnosis go hand in hand. So, not only do the students have Autism to deal with, but they must deal with ADD. This can be a challenge for those that try to educate youth with both of these diagnosis. A lot of times students will say they are thinking while they are zoning out, but if you prompt them usually they get back to the task at hand. Now sometimes students will use an avoidance behavior e.g. Licking a desk, getting up and walking around. It is important to help these students with these avoidance behaviors.
As always ask any questions.