This study examined the efficacy of a core kindergarten mathematics program, ELM, a 120-lesson comprehensive curriculum providing instruction in (a) number operations, (b) geometry, (c) measurement, and (d) vocabulary. ELM is designed to address the learning needs of all students, including at-risk students in the general education or Tier I classroom setting. The study utilized a randomized block design, with 64 classrooms randomly assigned within schools to treatment (ELM) or control (standard district practices) conditions. Measures of achievement were collected at pretest and posttest to measure student achievement. Students did not differ on mathematics assessments at pretest. Gain scores of at-risk treatment students were significantly greater than control peers, and the gains of at-risk treatment students were greater than the gains of peers not at risk, effectively reducing the achievement gap. Implications for Tier I instruction in a Response to Intervention (RTI) model are discussed. (Contains 4 tables.)