The California Teachers Association and United Teachers Los Angeles provide new evidence behind a familiar argument, that charter schools significantly under-enroll students with disabilities, particularly those with the most expensive impairments to treat, such as autism. The charter under-enrollment significantly shifted special education costs to districts, the study found.  Union researchers say their analysis is the first extensive comparison of enrollment and costs between charter schools and districts that authorized them. It examined the three districts with the largest numbers of charter schools – Los Angeles, San Diego and Oakland. The report details sources for the data and the methodology.