From what I am learning with my DS7, when a HG+/PG child is not challenged or interested in the topic/task it has a big impact on his/her attention span. My DS7 has "selective" attention. When he is really challenged or interested in a task, DH and I joke that a bomb could go off near him and he wouldn't notice.
However, most of the time at school this past year he was off task and didn't pay attention during class. I observed this first-hand because I volunteered in his classroom quite a bit. Also, as part of an evaluation done this past year by a neuropsych, they did a classroom observation where he did not know he was being observed. They found his male peers to be on task 70% of the time, and he was on task 35% of the time. The neuropsych does not feel he has ADHD, but thinks it is because 1. He is bored and 2. He has DCD which has a big impact on his ability to handwrite. If there is a task involving handwriting, he is a master at avoiding it.