In U.S. healthcare, SED is an acronym for serious emotional disturbance (or in some areas, severely emotionally disturbed).
Emotional Disturbance is one of thirteen disabilities outlined in the Individuals with Disabilities Education Act (IDEA). Section 1912(c) of the Public Health Service Act, as amended by Public Law 102-321 defines children with a serious emotional disturbance as those who are from birth to age of majority who have had a diagnosable mental, behavioral, or emotional disorder of sufficient duration to meet diagnostic criteria specified within the Diagnostic and Statistical Manual of Mental Disorders (DSM). For the individual to be deemed emotionally disturbed, it must be determined that the child's condition results in functional impairment, substantially interfering with one or more major life activities, such as the abilities to eat, bathe, and dress oneself, or the abilities to function effectively in social, familial, and educational contexts.
Students with SED are more likely to present externalizing behaviors (such as destroying property) than internalizing behaviors (such as withdrawal).