We evaluate a tracking program in a large urban district where schools with at least one gifted fourth grader create a separate "gifted/high achiever" classroom. Most seats are filled by non-gifted high achievers, ranked by previous-year test scores. We study the program's effects on the high achievers using (1) a rank-based regression discontinuity design, and (2) a between-school/cohort analysis. We find significant effects that are concentrated among black and Hispanic participants. Minorities gain 0.5 standard deviation units in fourth-grade reading and math scores, with persistent gains through sixth grade. We find no evidence of negative or positive spillovers on non-participants. Appendices are included. [This paper was published in "American Economic Review" v106 n10 p2783-2816 2016.]