For some diagnoses, there is a very negative stigma. ADHD for example. A child might go home from school and tell their parent "Guess what, the teacher says Abby has ADHD". So if that parent doesn't know Abby and has a choice between inviting Abby and another child for a play date, who are they going to choose? The child that might be "difficult"? Are they going to encourage their child's friendship with a child that has a behavioral disorder? Probably not.
Teachers who have worked with kids for years in the classroom are completely ignorant about ADHD. So to expect a label to produce positive reactions from yet other people who have NO exposure to it is unrealistic. I tell other parents about DD's ADHD only if they know her well. People always seem surprised because she is probably one of the quietest and most well-behaved kids in school.
