Noun, plural in construction but singular in usage. Originates from the acronym "Board Of Cooperative Educational Services," (or something similar), a 'special education' system often used to educate mentally-retarded people (frequently children, but occasionally brain-damage victims) as much as one possibly could, which, typically, isn't much. The acronym eventually became a term to describe any person so enrolled in one of these systems, and through logical extension, a an insult to suggest that they are as intelligent and mentally competent as a person enrolled in a BOCES system.
"Jim is such as boces, he can't even turn on the computer correctly."
"Our teacher is a real BOCES. It took him 20 minutes to get the VCR running."