Just a small point on some earlier points, achieving above average does not equal achieving the best you could.
I'll use myself as an example, I fall somewhat heavily and consistently into the introvert category - whether manufactured or not - and during my early schooling I was put ahead 1-2 years depending on which school I was at (I attended several due to moving) and given a great deal of limited student socialisation with high teacher interaction which ran roughly parallel with the general body. This was done primarily with teacher aides in larger classrooms where trainees were common and had the time to devote to this.
I averaged at 90-96% in my general testing up until late primary school and upon entering intermediate (11-12 years old normally, 10-11 for me) when I was basically shunted back into the standard classroom system - 25-40 students, connected rows of desks and an emphasis on socialisation. Within 6months of the environment shift my grade average dropped to around 60-70%.
This remained consistent right through intermediate and into early-mid college (13-17 normally, -1 for me) and it wasn't until late college where several of my teachers, specifically in biology, physics and english, let me simply sit in the back corner and chew through text books (and in the case of english, anything really) provided I participated in the group activities - IE speeches *shudder* and group research projects - and my grades in those classes shot up to back to high 80s, low 90s.
Obviously as a single set it is open to conjecture, however my point is that while I was allowed to work quietly and somewhat to the side my grades were consistently better, I also enjoyed the classes more and took more from it. I still had an above average score while in other classes and I still passed and went to university but I do feel that one environment suited me a great deal better than the other.
My point is just because someone 'does well' does not mean that the system is supporting them and their abilities, merely that they are doing well within it.
An obvious counterpoint might be 'if introverts are doing well despite this, does that mean extroverts are also being inhibited, but by the testing method rather than the teaching?' since several of my more extroverted friends and associates excelled at speeches in the same subject veins that they struggled with when performing written/researched based tests. Again small sample sets and deliberately vague, but a point I think should be noted.
Fell free to pick apart - just my personal observation..