In an elementary school, there are often four or five classes of each grade level. In our district, one of those classes is for children in the gifted program. If there aren't enough children in the gifted program in that grade level, other students who score high are added to fill up that class. Children are placed in the other classes by a combination of chance, parent requests and an intent to place a balanced number of boys and girls in each class. The school also tries to make sure that there aren't too many children who are likely to present behavior problems in the same class.
As a result, most teachers have to teach a wide range of academic abilities. Some children understand the material after hearing it once, while others need to have it repeated in different ways. How is a teacher supposed to make sure everyone learns what they need to learn when they are so different? I've heard teachers say that they "teach to the middle", meaning teach what the average student needs to learn. I guess this is the best solution given a wide range of abilities in one class, but I think those at the top and the bottom of the class get shortchanged.
Those at the top of the class are not challenged enough. For some, the boredom can create behavior problems. Teacher often keep these children busy with more work, but it's often just reinforcement of something they've already learned, or busywork. Doing 20 more math problems on a concept they're already mastered is not going to do anything but keep the kid busy and out of trouble. These are kids whose minds are ready for more, and we're just wasting their time. Many of these advanced kids learn that school is easy. All you have to do is show up, listen most of the time and you're there.
Those at the bottom of the class are always trying to keep up. Just as they're starting to grasp a concept, the teacher moves on to the next thing. Since they're at the bottom of the class year after year, they get the idea that they're not as smart as their peers.
Instead, what if they divided the children into classes by academic abilities? The teacher would have a smaller range of abilities to work with and could tailor the curriculum to that group. There would still be some at the high end and some at the low end, but no one student would be so far from the middle. There would be a better chance that a student who underperformed in one subject might excel in another, in relation to his class.
Those in the lower classes would get more reinforcement of the core concepts that they really need to master before progressing to the next grade. Those in the upper classes could move on to more challenging material once they've mastered a concept. Teachers could cover the next level of a particular topic, or add to the curriculum so that kids would be exposed to something they wouldn't otherwise get at school.
In a perfect world, each child would learn exactly what they are ready for. Unfortunately, we can't afford one-on-one instruction. Putting them together based on ability would be a step in the right direction.