Take a class of 30 children, they will have a wide range of intellectual abilities, ranging from borderline intellectually disabled to the highly gifted. Give them all the same task. Some will achieve well, and have no difficulty, one or two may not be able to complete the task without assistance. Take the same class the next day and have them perform the same task. Those who struggled will may perform better, those who completed the task the first time, will achieve the same result. Perform that same task for the next week or so, until the ones who struggled can perform the task. Now you will find that those children who initially had no problems with the task, have not even bothered to start it, and may well indeed be causing disruption in the class.
What I've described is what happens for most classes of children in most schools. The focus on ensuring all children reach some minimum performance in key areas has resulted in those children in the normal to high ability range are being shown that there is no value in being smart, as you are just going to have to do the same as everybody else anyway... so why bother?
The problem with Bright Kids is not that they never learn how to work hard on difficult tasks. It is that because they meet the minimum safety net requirements, they never are never even exposed to the difficult tasks, so is it any wonder that they grow to doubt the value of their abilities?
What I would love to see is every child in primary school be tested every year, for both potential and ability. This would allow Parents, Teachers and the students themselves to get an understanding of how hard the student is actually working. Are they really "smart" and bored out of their brains and therefore not meeting the academic requirements, or are they intellectually challenged, but through a lot of hard work, are achieving a reasonable standard?
Children do not all come from the same mould, why is it that we try to have them fit the same mould when it comes to education?
I'd recommend watching Sir Ken Robinson's iconic talk with respect to what you said:
Standardised testing does not test for ability to improve.
It's one thing if a kid fails because he didn't try hard enough. That can be explained to him. And it's a good lesson. But it's another thing to give him a standardised test and decide a priori what's he capable of. That just creates a strange value system where all kids, bright or not, may be denied an opportunity to show what they can do and they learn to think of ability as fixed. And it creates a lot of fascination with "innate" ability.
Is it true there's growing evidence that ability is perhaps not as "fixed" as we think?