The term "traditional classroom" brings to mind visions of a classroom of twenty or more children, all roughly the same age, listening raptly to the instructor or teacher, while all working diligently at their studies. The reality is far different.
In a traditional classroom environment, there are usually anywhere from twenty to thirty children and one teacher. In some cases, that teacher might have a "teacher's aid" or an assistant teacher. This is where the disadvantage starts- it is simply impossible for one person to give each student the one-on-one attention or instruction that may be required.
In a classroom of twenty to thirty children, there are going to be several children with differing learning styles and academic strengths and weaknesses. A teacher and/or teacher's aid is going to be woefully unequipped to handle any and all learning styles or weaknesses. And woe to the teacher who has a couple of students who present themselves as "problem students," a child who is difficult to manage or who disrupts the classroom is also going to take away time that the teacher could be using to help with one-on-one time with his or her students.
Which brings to light the next disadvantage of a traditional classroom. In a traditional classroom environment, it's not uncommon for children to become bored or frustrated. Some children learn better by visual means, others will learn better with auditory means, and still others are going to learn better with a hands-on approach.
It is virtually impossible for a single teacher to accomodate all methods of learning when he or she is responsible for teaching a large number of children. Thus, children who do not fall into the "traditional learning method" category are far more likely to "fall through the cracks," become bored or discouraged, or to display behavior problems in the classroom.
The traditional classroom environment works well for a large number of children, but there are many others who simply have difficulty learning in this environment, have difficulty interacting with peers that are strictly in their age range, or who require more one-on-one attention and time to grasp certain concepts.
The fact that a traditional classroom environment consists of one teacher and a large amount of students, in and of itself, is a big issue. But another issue that should not be ignored is the fact that children are seperated and segregated into classrooms filled with other children only in their own age-range.
For those who discourage homeschooling due to the "socialization" issue, there is another reason why the traditional classroom environment is still not the correct answer to socialization. Outside of the educational environment, and into adulthood, children are not always going to be around others their own age. It's important that they learn to relate to those not only their own age, but also those who are both younger and older.
The disadvantages of a traditional classroom are many. When it comes to children and education, there simply is no "one size fits all" answer, and attempting to force children to all learn in the same way is counter-productive to producing healthy, well-educated and well-rounded adults.