Suppose that you are asked to conduct a study

Suppose that you are asked to conduct a study to determine whether smaller class sizes lead to improved student performance of fourth graders. Further suppose that you can collect observational data on several thousand fourth graders in a given state. You can obtain the size of their fourth-grade class and a standardized test score taken at the end of fourth grade. Why might you expect a negative correlation between class size and test score? And would this negative correlation necessarily show that smaller class sizes cause better performance?