University of Florida:

However, the trend once known as “mainstreaming”— widely considered the best option for such students – appears to have stalled in some parts of the country, the study’s authors report. And a student’s geographic location, rather than the severity of his disability, often determines how he will spend his school days, the researchers say.
“We’ve known for a long time that students with MR (mental retardation) are better off educationally if they can spend at least part of the day in a typical classroom,” said James McLeskey, chair of special education in UF’s College of Education and an author of the study. “We’ve found that there are still lot of students who could be included in the general classroom but aren’t included.”
Before the mid-1970s, most children with mental retardation were completely segregated from other children in the school system, if they were formally educated at all. Society widely viewed these children as uneducable, and those who did attend school were sent to institutions solely for children with mental retardation.