At best, a teacher can keep the students in for recess, to try and work individually with them on lessons they are struggling with, but most students are not very willing to miss their break. As well, if you see from a few tests that one student is struggling with spelling, another with science and another with math, it is impossible to work with several students, on varied lessons, in the ten minute recess break given to students.
As well, as classroom sizes grow, so does the burden to classroom teachers. Teachers not only have more work to prepare, and more marking and evaluating, but they also have to keep a classroom of say 35 students quiet, in order to teach a lesson. And as anyone familiar with the teaching profession knows, it is hard to keep that many students quiet and focused, in order to teach a lesson. All you need is one student who struggles to focus to disrupt the class and keep everyone distracted. And with 30 or students, it is likely that there will be at least a few class clowns who can’t sit still. As well, when you have a small room with 30 or more students crammed in it, it will become crowded and uncomfortable for students, making them even more restless.
Overcrowded classrooms impact both students and teachers negatively, and they are putting students learning at risk, making it easier for struggling students to fall through the cracks. Instead of helping to eliminate or at least reduce the number of students who under-perform in school, overcrowding is only making the problem worse.