Background: Magnet schools emerged during the 1960s as part of an effort to desegregate schools across the United States. Their primary purpose is to provide appealing educational settings to induce voluntary desegregation through parental choices (George & Darling-Hammond, 2021). Some of these magnet schools operate at a regional level to address interdistrict desegregation, and Hartford, Connecticut stands out as a notable example. Despite some studies demonstrating the potential of Connecticut interdistrict magnet schools (IMSs) in fostering desegregation (Quick, 2016; Orfield & Ee, 2015), there is still a gap in understanding how they impact diversity among a range of nearby school options. While the recent study by Alcaino and Jennings (2020) suggested that magnet school enrollment growth had little effect on county-level racial segregation and private school enrollment nationwide, no specific study has delved into Connecticut's unique dynamics. This study aims to provide insights into compositional enrollment dynamics between IMSs and both traditional public (TPSs) and private schools, especially Catholic schools that serve a more historically disadvantaged student population. The findings will inform policymakers concerned with offering school options to promote desegregation, not only in Connecticut but also in other states. Research Questions: Our specific research questions include: 1) To what extent does the opening of IMSs explain the change of enrollment in proximal TPS and Catholic schools? 2) To what extent does the opening of IMSs explain the change of racial and economic diversity in proximal TPSs and Catholic schools? 3) Does the impact fall differently in schools located in Sheff region and those eligible for Title-I? Setting/Intervention/Population: As a part of outcomes from 1989 Sheff v. O'Neill lawsuit, the Connecticut State Department of Education (CSDE) introduced the Comprehensive Statewide Interdistrict Magnet Plan in 2009. By the 2016-17 school year, they had established 91 IMSs, including a significant portion of Hartford's minority student enrollments (CSDE, 2016). This backdrop creates a great opportunity to learn about the impact of opening magnet schools on student composition. Our key intervention of interest is the opening of the first IMS within 2.5 miles radius from TPSs and Catholic schools. Data: The main data source in our study is the National Longitudinal School Database (NLSD), which includes public and private schools' geographic location and annual enrollment by race, and free-or-reduced lunch eligibility (Carroll et al., 2023). To distinguish IMSs from regular magnet schools, we manually checked reports published by CSDE (2016). Our main datasets span from 1998-99 through 2019-2020 school years, separately for K8 and high schools. It allows us for a more precise impact analysis by capturing the first IMS opening that share at least one overlapping grade. Our final sample for K8 captures 789 TPSs, 122 Catholic, and 58 IMSs. High school sample includes 143 TPSs and 26 IMSs. Figure 1 presents the location of schools included in our sample. Research Design: We employ staggered Difference-in-Differences and event study approaches (Callaway & Sant' Anna, 2021) to address our research questions (see Equation I). Our estimates compare schools treated by IMSs opening with those never or not-yet treated. The key outcomes of interest include 1) school-level enrollment, 2) entropy index of racial diversity, 3) racial compositions of white, Black, and Hispanic students, and 4) the proportion of low-income students (eligible for free or reduced lunch; FRPL, TPS only). We then focus on two potential sources of heterogeneity in the impact -- whether a school is in Sheff region (Hartford County), and whether a school is eligible for Title-I -- to determine whether the policy's intended target is functioning. Main Findings: Overall, the impact of IMSs openings on proximal TPSs' racial and economic diversity significantly differs by Sheff and Title I status. First, only the treated TPSs located in the Sheff region and eligible for Title I status experienced an enrollment drop following the opening of IMSs within a 2.5-mile radius, particularly high schools (Figure 2). Regarding racial composition dynamics within these schools, the white share increased, the Black share remained relatively steady, and the Hispanic share decreased (Figure 3). Second, although there is a limitation regarding the pre-trend assumption, the percentage of FRPL students in TPS high schools significantly increased following the opening of IMSs (Figure 3). Third, the total enrollment of K8 Catholic schools also decreased, but there was no significant difference in patterns based on whether they were in the Sheff region or not (Figure 4). The white share decreased, while the Black and Hispanic shares increased, showing an opposite pattern from TPSs (Figure 5). Conclusions: This study expands upon existing literature investigating the impact of free public school options on racial and economic diversity, with an emphasis on interdistrict magnet schools, thus moving beyond the predominant focus on charter schools. Our findings suggest that Hispanic students in Sheff Title I TPSs, as well as overall Catholic school students (historically serving a large body of low-income students), who are the target of IMSs policy, may opt to enroll/transfer to those schools. Moreover, our research highlights that these enrollment patterns are particularly pronounced in TPS high schools, where students may be more inclined towards longer commutes for magnet schools offering specialized programs that align with their career pathways or college goals. These findings underscore the necessity for future research employing mixed methods to delve deeper into the decision-making processes of students and parents when selecting schools. Understanding how these decisions vary across racial and economic demographics will be crucial for informing policies aimed at promoting desegregation and equity within public school choice options.