The more we learn about the complex process of attention, the more we know that behavioral checklist-diagnoses of children for ADD and ADHD is not the answer. When children aren't paying attention in class, the possibilities are as complex as the brain itself - for some children it does seem to be a global impaired sustained attention, but for others it is auditory attention only or visual attention only, problems with visual or auditory perception (or both!), problems with shifting attention, or problems with feature-based attention. Feature-based attention is the topic of the link below. In this study, both frontal and parietal (spatial cortex, a place where sensory inputs integrate, images too)areas were important, but the parietal lobe especially for selecting the task-relevant feature and ignoring details not relevant to the task. The task here was a visual sorting task involving number estimation.
Figuring out the true nature of "what it takes to attend" is essential before we are really able to go about troubleshooting it. In children particularly, pathways and connections are actively forming. "Training the brain" is really just understanding what specific tasks the brain has trouble with, and matching the right targeted practice or education to build connections.