I believe that if Plato or Socrates would visit our century and see what we call education they will be either horrified or they will laugh hysterically. Our society since many centuries ago decided to provide education to all kids which is not a bad thing for itself, especially if we remember that once it was reserved only for the upper class and royalty. The problem is that in order to provide "education for the masses" we adopted an industrial model with the same standards of efficiency, service and quality control.
The moment you put 30-40 kids in a single class you necessarily lose the ability to reach each one of them, introverted or shy kids will disappear and "fall under the radar", the time the teacher will have to help, assist or dedicate to a pupil will be extremely limited thus making the whole learning experience almost ineffective. This model can serve a class of very bright, well-disciplined kids although it will not be the best even for them and their advantages will not be properly attended or encouraged. But in a regular class you have usually all levels of learning abilities and of course, the same about discipline so in such class a teacher will probably lose a big percentage of the kids unless he is ready to work till a point of exhaustion.
Let's forget for a moment what I mentioned in the last paragraph. A kid sits in a classroom and what he usually receives there is a lecture of some type that has a purpose for him to memorize material that the pupil may not find interesting (and let's disregard for a moment the kids with concentration disabilities) the same is done in most subjects he learns that may sum to an average of 10-12 different subjects. This is not an easy task to do. To make matters even worse, since teachers are required to measure their advancement, they fill the spaces in between with tests and quizzes, adding to the pressure already existing.
Is this really learning? I seriously doubt it. We organized a system that more than serving its purpose it serves itself, we all know from personal experience that it is not good but since everybody do it then we will do it too and so our children and future generations. In the system as it is today many kids don't like to assist school and see it as a source of stress, many drop out, especially in high school when they feel independent (or rebellious) enough to do so.
Learning as the ancient masters describe it is built upon attending the level of comprehension of every pupil, the attempt to bring all pupils to the same level is artificial and futile. The teacher should not see his job as part of a system of data gathering, if a teacher see his duty as such then he can be easily replaced by a mechanical device or a computer. A teacher should instigate curiosity and encourage his pupils to research, to think and guide them to the answers they seek, learning is a process, it has a rational and a logic that cannot be recreated or substituted by a cold impersonal routine of repeating and memorizing. I know that it is not easy, it requires a lot of resources, funding and facilities but above all, it requires a radical change in the approach.
Another major obstacle is the discipline problem that is part of most educational systems around the world. I mention it here because it is relevant to the question on relation to things that should not be part of the system if we want to build a system that serves its purpose of learning. Try to remember your school years, many of us experienced problematic pupils in our class, sometimes violent which made learning so much more difficult. In a class with 30-40 kids, a few of them can turn any learning impossible. My question is: who decided that all kids should learn under the same conditions and at the same level? Why should a teacher dedicate some of his limited time trying to calm-down these problematic kids? If you ask many representatives in the system (and I did) they will tell you that it is a necessity to comply with the existing laws and rules.