... omigosh I never noticed that!!
*starts turning 48s around in her head, being quite stereotypically aspie about it*
AS is a developmental disorder. It's a bit like a learning disability, and many schools categorize it that way. The main meaning of the developmental disorder category is that it's the result of a brain that develops differently from the typical.
Studying AS in a class about mental disorders makes sense, though, because it is in the DSM and included in the mental section of the ICD--along with other brain-wiring things like ADHD, learning disabilities, and mental retardation, which are also not "mental disorders" in the strictest sense of the word. Psychology classes often cover a broad group of diagnoses, which include not just the classic emotion-related psychological diagnoses, but other sorts of differences in the way the human mind can work, some of which, like autism, are present from birth and represent that particular brain's healthy state.