Reducing Class Size in the Early Grades Have a Positive Effect:
Education researchers suspect that class size reduction in the early grades helps students to achieve because there is a greater opportunity for individual interaction between student and teacher in a small class. Teachers generally have better morale in a small class, too, and are less likely to feel overwhelmed by having a variety of students with different backgrounds and achievement levels. As a result, they are more likely to provide a supportive environment.
In the early grades, students are just beginning to learn about the rules of the classroom, and they are figuring out if they can cope with the expectations of education. If they have more opportunity to interact with their teacher, they are more apt to feel like they can cope.
This theory would also explain why lowering class size in the upper grades may not have the same affect on achievement. Students in the upper grades, who may not have had the benefits of a small class in the early years, have already formed their habits, good and bad, for coping with their classroom environment. Simply reducing the class size at this level may not be enough to change their ways.