This paper analyzes households' response to the introduction of intra-district school choice and examines the impact of exercising this choice on student test scores in Pinellas County Schools, one of the largest school districts in the United States. Households react strongly to the incentives created by such programs, leading to significant changes in the frequency of exercising alternative public schooling options, as well as changes in the composition of the "opt out" students. However, using "proximity to public alternatives" as an instrument for opting out of the "assigned" public school, the author finds no significant benefit of opting out on student achievement. Also, the author finds those who opt out of their default public schools often perform significantly worse on standardized tests than similar students who stay behind. Results suggest that the short-run detrimental effects of opting out are stronger for students who opt out closer to the terminal grade of the school level, yet weaker for "disadvantaged" students, who typically constitute the proposed target of school choice reforms. (Contains 5 figures, 14 tables and 33 footnotes.)