The debate regarding the effectiveness of educating students with disabilities in inclusive settings versus traditional Special education settings to yield increased levels of achievement has been ongoing in the American education system for decades. This action research study was conducted at a sixth through eighth grade public middle school in the Southeastern United States employing a quasi-experimental research design. The objective of this quantitative study was to discern the most effective mathematics LRE model for middle school students with disabilities to close the achievement gap between them and their non-disabled grade level peers. The mathematics scores of a convenience sample of 86 students with disabilities from various eligibility categories were obtained from two administrations of the Mathematics Inventory, a universal screener. The research participants were divided into two groups per grade level. The experimental group that received their primary instruction in a general education mathematics co-teaching setting and the control group that did not. The mean growth scores of the experimental group were compared to the mean growth scores of the control group using independent samples t-test on each grade level. The results from the independent samples t-test showed no statistical difference between the means of the grade level comparison groups in the sixth, seventh or eighth grade at the research site. [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.]