Here are the key elements of a research proposal on this topic:
Participants: The sample should include preadolescent children (ages 8-12) from both urban and suburban elementary schools. It should include an equal number of children who participate in organized sports and those who do not. A sample size of at least 100 children in each group (organized sports vs not) would allow for adequate statistical power.
Measures: The dependent variable is standardized test scores. These should be obtained from official school records for each child's most recent standardized test (e.g. IOWA, SAT, ACT).
Procedure: Parents would complete a brief survey indicating whether their child participates in organized sports outside of school physical education classes.