I am E.T little sister. Do you want to be my Earth friend?

When I arrived here, I didn´t understand anything. Here, on Planet Earth, people is educated in big buildings which are called "Schools". There are one person, an adult, a big one person, who has the Knowledge-or, at least-everyone thinks that he or she Knows. On Planet Earth everyone call him-or her- "the teacher". This person shows and explains the subjects to a group. This group is formed by little persons which are called "pupils". They learn what the teacher explains. All of them are in different "classes"-those are like little or big rooms in a house. The house is School.
My friend, could you explain me why Education on the Earth is organizated on this way? Why are they all together? Why is there only one teacher for a fourty or fifty pupils group? Why?...
-The Earth friend answers-"Because is easier for the society this kind of organization. It´s cheaper, it´s easy to organizate all group and transmite the Knowledge, it´s more simple and better on this way".