The first and most important reason. Teachers teach students in a class is not around to students 1 classroom has more than 20 students were taught to not make some students don't understand the lesson.
The first and foremost reason Teaching students in one classroom, students are not evenly than 20 students were taught not thorough. Some students do not understand.
The first and most important reason Teaching students in the classroom, the students 1 incomplete class with students than 20. Teaching is not thorough, some students don't understand the lesson.