As the number of students in a classroom grows, so do the negative effects to both the students and teachers. A teacher can only devote so much attention to one student, when he or she has to divide their attention among 20 or more students in the class. As a result, when the number of students in a classroom rises from say 20 or 25 to 30 or 35, it is impossible to give all the students the attention that they need and want.
With so many people vying for the teacher’s attention, any student who is struggling and is reluctant to ask for help during class time, as many of the other students are doing, is going to be inadvertently left behind. When 10 students have their arms up and are seeking your help, you cannot possibly help all those students and then still get a chance to go around and see how the students who are not asking for help are doing. Therefore, until students are tested and assignments and tests are marked, a teacher might not know that a particular student, who did not ask for any help, was struggling with a particular lesson.
Even once a teacher realizes that a particular student or students did not grasp some of the lessons; it is already too late, as that particular lesson will have already been taught, and there simply is no possible way that a teacher can go back and reteach an entire lesson for a few students.