I'm going to paint a little picture here... A picture of a couple of effects, with one cause. I did a post a hundred years ago about "Lowest Common Denominator". Alas, I can't find it on a site search, so the gist of it is that in our Public Education System, the emphasis has migrated from Common, to Lowest. I contend that "Average" (or Common) has been skewed down to incorporate under-achievers, so that they may feel better about themselves. This is something that you see in little-league sports competitions all the time. They don't keep score (but the kids do), everyone gets a trophy for participation-but no one gets one for winning, etc. Now apply this concept to academics.
With the endless repetition of base concepts prevalent in current Public Education, (I experienced it myself, and I'm Thirty-Six years old) I can see a certain propensity for children to get bored. Not just the fourth repetition bored, but the thirteenth repetition BORED. At a certain point, all children will reach this threshold, and start acting in ways that bored kids will act.
It used to be that children were given lessons, they were Tested on their knowledge of the information presented, and then class is resumed at a level based on the results of the tests. The Average results. Now, it seems that the class restarts at the level of the worst test results. The rest of the class may be doing long division, but one student may be stuck on multiplication (aka: basic addition), so everyone is stuck back on simple multipliers... For as long as it takes for the stragglers to "Get It".
Now there are two basic kinds of kids... the ones who are extroverts, and the ones who are introverts. When a child becomes bored, they will lean on what comes natural to them. They will either "Show out" or "Close up", based on their own proclivities. They will react in whatever way comes "naturally" to them.
If a child is Extroverted, they will become more emotive in order to attract more attention. If Introverted, they will find something else that interests them, other than the things currently being studied. Luckily, there's a few diseases out there to take care of these issues.
ADHD for me, equates to "I'm bored so I'll just do something to attract attention to myself". Autism for me, equates to "I'm bored so I'll just read this science fiction book until something interesting happens in class".
These things aren't hard and fast, but they are there. Of the two most over-diagnosed childhood afflictions, ADHD and Autism top the list. If there's less than 85% of people diagnosed with either of these afflictions that aren't scored above average, I'd be genuinely stunned.