A lot of people out there have some form of learning difficulty, and a surprising number of them are treated as intellectually sub-normal by people who should know better. The classic sign that someone has some form of learning problem is a significant difference between how bright they seem when you talk to them compared with how bright they seem in their work.
Classic comments that these people recognise include "could try harder", "they are lazy", "you are an idiot", etc. most of these comments can be found comming from the group that has the least excuse for using them, teachers. These comments are an obvious sign that there is a real problem there, and the solutions can range from the simple to the extremely difficult depending on what the cause of the problem is.
Causes include being short sighted, and placed at the back of the class, which can be resolved by getting the individual fitted with the right set of glasses, and sitting them at the front. Another often missed problem is being slightly hard of hearing, so you can't quite hear what the teacher is telling you to do. This can be solved by getting the individual fitted with a hearing aid. Both of these problems are blatently obvious things to check for when you are getting comments along the lines of "the work he does is very good, but it is not the work he is asked for".
Another obvious thing to look for is the simple question of can they read and write? This is often completely missed with the intelligent, as they are clever enough to come up with coping strategies that effectively cover up this basic problem.
Then you get on to the much harder problems like dyslexia, dysgraphia, dyspraxia, etc. these can be helped if you catch them early, with lots of extra work, but often they are not caught until the individual has gone to jail (where 50% of inmates are undiagnosed dyslexics) or until they try and get a job. If not diagnosed until this stage, then the individual can end up with long term problems with linguistic skills like spelling, and sentence composition, even when they are verbally quite erudite.
You also have a set of problems like Attention Defecit Hyperactivity Disorder (or ADHD for short) which require long term medication, and carefull management of dietry components to make sure nothing makes it worse.
None of these problems should make it out of your first four years of schooling, but for various reasons, they tend to be missed until the individual in question passes through the hands of that rarity in the teaching profession, the person that everyone recognises as the good teachers at that school. These characters are exemplified by the will to put themselves out for the sake of the pupil, often going much further than even the ordinary teachers think is reasonable, to make sure that the pupil leaves the learning environment a more fulfilled individual than when they arrived.