More accurately, best practice dictates that the symptoms should be present across settings. Also, that there should be multiple raters and methods of data collection, including direct observation. (I make sure to do my own direct observations in at least some of the same settings from whom I receive ratings.) Unfortunately, there are practitioners out there who do not follow either best practice guidelines or good clinical (common) sense. In my experience, it is whoever wants the child out of their hair who pushes for the Dx (often parents, often teachers/staff). I have seen the discrepant reports work both ways. Also, many pediatricians are not qualified to make this diagnosis, either from lack of training, insufficient time/data gathering, or both. Yet they routinely do so, and put kids on psychotropic meds.
Note: I'm not sure what manual you saw, but none of the ones I have imply that parents are less credible. School personnel will automatically lean toward that assumption, though, because the teacher reporters are people they know personally, and with whom they have relationships. Additionally, I think it is safe to say that there is greater diversity in home environments than there is in school environments, which makes the meaning of parent ratings more challenging to interpret than that of teachers. (I mean, some of us think it's okay to do oral algebra problems while kicking a ball into a soccer goal, while others would consider that excessively high levels of activity.)
This is why I think the most productive solution to this problem is to cultivate balanced relationships and cordial communication between parents and school staff as much as possible. ... I know I'm being an idealist, but one can but try!
