I think it's also the new "popular" diagnosis. My cousin is fighting an autism label on her child. The school feels that he is autistic because he "fixates". What he does used to be called an attention span.
It is regrettable some people, like at the school you speak of, consider themselves physicians. They are not licensed to practice medicine. Perhaps they should be held legally accountable?
I do not think autism is one of those pop-syndromes recently in fashion. I have researched this to some great extent...
...a genetic disorder related to a variant allele of HOXB1, a gene on chromosome 17, possibly regions on chromosome 15, chromosome 7 and chromosome 9. A remarkable and detailed report on the etiologies, brain mechanisms, and neuropsychological phenotypes can be found in Diagnosing Learning Disorders; A Neuropsychological Framework (which may be now outdated). This following description is a valuable bit of information from that 1991 book:
…Briefly, existing evidence supports the conclusion that autism is familial, heritable and genetically heterogeneous. Among the possible genetic subtypes are multifactorial inheritance, autosomal recessive inheritance, X-linked inheritance and nonfamilial chromosomal anomalies.
In any case, your cousin should seek legal advice if she feels school officials (usually pretentious liberals) are creating a psychological danger to the child.