An achievement gap is defined as a significant difference in academic performance between two groups. Massachusetts Comprehensive Assessment System (MCAS) data indicate that, in Crestland Public School District (pseudonym), there is a clear achievement gap in mathematics between students with disabilities and those without disabilities, specifically at the elementary level. The percentage of students with disabilities achieving MCAS mathematics scores at the proficient or advanced levels is significantly lower than the percentage of students without disabilities. There are comparable schools in the state of Massachusetts where the percentage of students with disabilities scoring proficient or advanced is much higher than in Crestland. Studies that have looked at the improved academic achievement of students with disabilities have identified common practices. Among these practices are: (i) curriculum alignment to standards; (ii) inclusion of students with disabilities in general education classes; (iii) type of instruction; (iv) collaboration among faculty; (v) use of data; (vi) culture of the school; (vii) focused professional development; and (viii) supportive leadership. This study focused on examining the ways that schools have been successful in increasing their students with disabilities achievement on the mathematics MCAS test in third and fourth grade. Leaders and teachers from two comparable schools in two other districts, which have succeeded in improving the achievement of students with disabilities, completed surveys and were interviewed to gain insight into how Crestland might improve the achievement of its students with disabilities in mathematics. Findings indicate the prevalence in both schools of a collaborative culture focused on the examination of student data, and data-informed instructional decision-making. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]