The reasons students do or do not learn are tied to many factors, some beyond the control of even the most dedicated teacher. Many students come to school ready to learn, and having been exposed to numerous enriching experiences as pre-schoolers; their parents have read to them, enrolled them in pre-school programs, and generally exposed them to a stimulating environment from birth. However, sadly, many students come to school having had none of those experiences. There are many, many children enrolled in America's schools who have received little or no intellectual stimulation at any level in childhood. While some children may start school already reading, others of the same age may have not even learned the alphabet yet. This is the beginning of what is sometimes termed "the achievement gap." Unfortunately, that gap tends to widen for some children, and by the onset of adolescence in the absence of support from home, many kids have lost interest in school, an endeavor in which they have been disadvantaged from the start.
Educators commonly say that they believe all students can learn, and this is true. However, the issue isn't if students CAN learn, it is whether they WILL learn, and another problem facing educators is the student who won't do anything in school. These students are frequently not engaged in the teacher's instruction, and assignments are not completed, and/or handed in. Another problem is the student who is not engaged in, nor paying attention to, the instruction, then either expects the teacher to repeat the entire lesson for him or her individually--which some teachers will do--or just attempts to slap down anything on paper, regardless of quality, to hand in so he/she can say they turned everything in. In these days of rampant grade inflation, it is not unusual, especially in grade school, for a student who has always turned everything in, or most things in, to get all A's. Sadly, the teachers who give all A's are often perceived as the best teachers; when a student arrives in a classroom where a teacher has high expectations, the student will struggle to meet them, receive lower grades, and then, unfortunately, many parents will assume it is the teacher's fault because "he's never gotten anything but A's". As an educator for 15 years, I believe this may be one of the most serious problems in America's schools, and many people don't see it as a problem. You simply cannot give kids all A's and continue to pass them on through school when they cannot meet a standard of quality; yet it happens every day in thousands of classrooms, many times causing the teacher of high expectations to be vilified and seen as an ogre.
As educators, however, we must also heed the research that says that the single biggest impact on a child's learning is the teacher in the classroom. So, while even the most gifted teacher cannot change external factor's in a child's life, quality does matter. Additionally, there's plenty of research that shows that children WILL rise to higher expectations if we insist that they do. That is one reason why the issues of grade inflation and lower expectations are so troubling to teachers who are trying to maintain a standard of quality. At the risk of oversimplifying a very complex issue, it can probably be stated that home involvement or lack thereof, and school/teacher quality are probably the biggest factors impacting a child's learning in most cases.