Intelligence is not a fixed entity, but believing that may make it so
Research in the field of social psychology suggests that people have two different basic beliefs about intelligence. A few people fall into a middle ground between the two basic beliefs but the majority tend to fall into one or the other set of beliefs. 
One group tends to see intelligence as a fixed entity that you either have or don't have; that you are born with a certain amount and that amount can't be changed much throughout life; , that you either are an Albert Einstein type or you aren't. This belief is somewhat true in that research does suggest that intelligence is 40 to 80% due to genetics - how smart your parents are does correlate with how smart you may be. , However that leaves up to 60% of intelligence due to your own health status and work effort. If you believe that you will never be as smart as Albert Einstein so it just isn't worth trying to improve or study, then that is a self-fulfilling belief. Someone who doesn't study is unlikely to improve their skills or to stick to a difficult problem long enough to solve it.
Albert Einstein wasn't a straight A student but that didn't stop him from sticking with his field of interest and breaking new ground in the understanding of physics. But he probably wasn't as smart as Jean Liedloff was in the field of child rearing where she broke new ground in the understanding of natural parenting strategies. And Jean Liedloff probably wasn't as smart as Albert Einstein was in the field of physics but that is okay. We all are better off for having experts in a variety of fields of study.
Jean Leidloff is less well known then Albert Einstein but she did go where few before her had gone - and she stayed for a few years. Jean is best known for her book, The Continuum Concept, published in 1975. She described her time spent living with a South American tribe and her observations of their parenting practices. The book formed a basis for the attachment parenting movement.
The second basic theory of intelligence is that intelligence is incremental - that intelligence is something fluid that can be changed and increased with more study and effort.  This core belief is associated with students viewing learning as a fun self challenge. Students with a more incremental theory of intelligence are more likely to choose challenging learning tasks and to avoid tasks that they have already mastered (super boring, man). Students with a more entity theory of intelligence may be more likely to choose tasks that they will be able to complete easily as a way of proving to themselves how smart they are or to show others how smart they are. Students with the entity theory of intelligence may be more likely to feel threatened by other students who do well on a test or project; while students with the incremental theory of intelligence primarily judge their progress against their own previous work - with intelligence as a fluid changeable trait they are eager to learn and challenge themselves against themselves. 
Intelligence is fluid over the whole lifespan. Reaction times may be faster in our teens and twenties but social skills continue to improve on average with each decade of life. 
In the U.S. the No Child Left Behind legislation greatly increased the number of tests that young children are made to take and teachers can lose their jobs and schools can lose their funding if children don't perform well on the tests.  The policy may be leaving all the children behind by teaching them the entity theory of intelligence - that intelligence is something that you can measure with a test and that you either have or you don't. The entity theory of intelligence is associated with an increased risk of giving up when faced with unfamiliar, confusing, or difficult work.  More children are leaving the school system before they graduate, that also can leave them behind. [11, 12] Teaching to the test may help bring some children up to average but it may also be leaving the self-challengers in a state of mind numbing boredom and leading them to dropping out; or to only working for the easy 'A' instead of working to their maximum capability; or to acting up in class and becoming a behavior problem.
We need people with a variety of types of interests and skills and who see challenge as something fun and worth the work. The incremental theory of intelligence is a more accurate reflection of how our brains work over the decades. Testing young children every year of their early lives may be fostering the more inaccurate entity theory of intelligence. The good news is that children can be taught the incremental theory of intelligence simply by encouraging more projects with learning goals rather than performance goals. 
A learning goal might sound something like: "This task may be a little challenging but that is okay it will help you learn a new skill and the grade isn't important." And a performance goal might sound something like: "This task will be graded and your grade says something about your level of intelligence" --> and in the era of No Child Left Behind and Race to the Top programs --> "and your teacher's job and your schools budget may be affected by your grade.".
Young children's core beliefs may be harmed by the frequent testing that has become standard and high stakes that have become associated with the tests, in my opinion at least.
/Disclosure: I am a nutritionist. Disclaimer: Information presented on this site is not intended as a substitute for medical care and should not be considered as a substitute for medical advice, diagnosis or treatment by your physician. Please see a health professional for individualized health care services./
Carol S. Dweck, Self-theories; Their Role in Motivation, Personality, and Development, (Psychology Press, 1999, Ann Arbor) 
7/25/2018, A book from a more recent post reviews the history of study of intelligence and intelligence testing - it was to help find school age students that needed extra help and is not representative of all types of intelligence that adults need to be successful but it remains somewhat helpful for predicting ability to learn in school:
Richard J. Haier, The Neuroscience of Intelligence, (Cambridge University Press, 2017, New York), http://www.richardhaier.com/the-neuroscience-of-intelligence/ (1)