Dianavan, I'd be surprised if there were educational studies indicating that children with ADHD have a harder time learning as a result of medication. I've seen many children go from D's to A's in their schoolwork after starting medication, and they are clearly able to learn and use new concepts.

I wonder if the students you're concerned about might have had learning disabilities in addition to their ADHD. Sometimes a child's hyperactive behavior is what is most noticeable, then when the ADHD symptoms are improved with medication, in effect he is able to sit still long enough for the teacher to discover that he also has difficulty with some aspects of learning. The good news is that he's also able to sit still and pay attention to the educational interventions that can help to overcome or compensate for the learning problems.

Now, before someone jumps in and says that "these kids don't have learning disorders, they're just lazy," or normal, or whatever, let me hasten to add that there are a number of very impressive studies of brain function, using functional MRI scans (i.e., scans of a person's brain while he or she is engaged in a mental task), that show definite differences in the activation of brain centers and pathways between, for example, people who are good readers and people who have a reading disorder. If you're interested, or skeptical, I'd suggest you check out a book called "Overcoming Dyslexia" by Sally Shaywitz.   

Saying there is a "lack of dopamine in the brain" is an oversimplification, though it's a useful one. (The neurotransmitter norepinephrine is also implicated in ADHD, as well as dopamine.) Doing a blood test to look at dopamine levels would be useless. Brain cells use only a handful of different chemicals to "talk" to one another, but the combinations of connections produce billions of different messages. It would be more accurate to say that the neurons in the brain centers that affect selective attention and executive function seem to be working inefficiently, perhaps because they do not make enough dopamine and/or norepinephrine, or it's destroyed too quickly in the synapse before it has a chance to send its message. When we give medications that increase the amount of available dopamine and norepinephrine, those centers work more efficiently.

The adverse effects of these medications, such as appetite suppression and difficulty falling asleep, result from overactivity of other brain centers, which already have enough of the neurotransmitters, and don't work as well when they have too much. Ideally, we would be able to design a medication that would only be taken up by the problem areas in the brain and no others. But we're a long way from that.

Aloha,
Mark