Could it really be this simple?

Fund Small Class Sizes and Let Teachers Teach?

From Class Size and Students At Risk: What is Known?...What is Next? - April 1998

The issue persists because of the tension between the research findings and the cost of implementation. A great deal of empirical data have been collected. However, they have so far been less than convincing and not consistent enough to justify the expense of the additional classrooms and teachers that would be required. Targeted remedial programs are generally less costly and easier to deploy. They tend to be adopted for a portion of the school day to address learning problems in one or a small number of subject areas. In contrast, maintaining small classes throughout a grade level or school requires pervasive organizational changes. Of course, proponents would argue that the benefits are also pervasive--being realized throughout the school day and affecting the entire range of school subjects--unlike the band-aid approach of experimenting with one targeted program after another.