Since implementing No Child Left Behind, schools have improved student achievement while also preparing students for the 21st century. Schools continue to strive for 100% proficiency in all subgroups by 2014, but achievement gap exists for students with disabilities. This study used a causal comparative research design to test the concept of co-teaching by comparing types of placement for students with disabilities and the effect on student achievement. For this study, placement was considered where the student received instruction in either a general education class or a co-taught class during 9th through 11th grade. Student achievement was based on the 11th grade Pennsylvania System of School Assessment results for reading and math. This study examined the results from southwestern Pennsylvania high schools for the 2011-2012 school year. The independent variable was student placement in co-taught classes or general education classes. The dependent variable was student achievement based upon the state test results. A causal-comparative design matched 67 math and 68 English co-taught participants with non-co-taught participants based upon gender, disability, and full scale IQ. Analysis of variance was used to determine if the different placements had differential effects on the students' state math and reading scores. Math and English pairs were analyzed separately. The results of this study found that there was a significant difference in mean scores for class placement in both math and English classes. The students with disabilities placed into a general education class scored significantly higher on the respective 2012 PSSA tests. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]