My most recent post in the Special Education Law 101 series now running on the blog concerned least restrictive environment. LRE is one of the most misunderstood concepts in the field of special education law. Some people in audiences where I have given speeches are amazed that the words "inclusion" and "mainstreaming" are not in IDEA or the regulations.
Mainstreaming is a movement or a goal, but not a legal requirement. There is clearly a preference for mainstreaming under IDEA, but it is not required that every child be in a regular education classroom. The requirement is the least restrictive environment appropriate for an individual child.
Last summer I was on a panel at a conference addressing this issue, and the question was asked does the FAPE requirement trump the LRE requirement? The panel agreed that FAPE does trump LRE. I need to revise my answer in retrospect. FAPE does not trump LRE. FAPE trumps inclusion if they are in conflict. But FAPE and LRE cannot technically be in conflict, because the LRE requirement only requires mainstreaming to the extent that it is appropriate for the child. If a child cannot be successfully educated in the regular classroom with the use of supplementary aids and services, LRE is someplace else. So my revised answer is that FAPE does not trump LRE; they stand as the twin towers of special education law!