Final answer:
States have grouped schools by income to reduce educational disparities and promote equality and economic growth by ensuring equitable per-student spending and opportunities.
Step-by-step explanation:
Some states have grouped schools based on their income levels primarily to address educational inequalities and ensure that all students have access to quality education. Specifically, the sentence that explains why states might group schools based on income is: "It reduces the disparities in educational outcomes."
This goal is aligned with the concept of equitable per-student spending, where public policy aims to provide a reasonable opportunity for each child to attain an economic niche in society, regardless of their socioeconomic status.
The allocation of resources in this manner attempts to mitigate the performance divide caused by socioeconomic disparities, as demonstrated in the Coleman Report of 1966, and is further supported by the belief that a well-educated population can help the economy to grow and contribute to both equality and economic growth.