School rating scales carry a lot of weight for two reasons.
1. ADHD diagnosis requires that the symptoms be present across environments. So if it is only seen in one, it is not likely ADHD.
2. The research suggests that teachers are generally able to give more valid responses than parents (I would need to re-read the articles to remember just home much.) The researchers have hypothesized several reasons for this. The main one is that teachers have observed plenty of other children the same age to reference. (A general education elementary teacher with three year’s experience with class sizes of 25 have roughly observed the behavior of 75 students for 180 days per year for about 3-4 hours a day in academic, social, unstructured, and structured settings. This should include moderately disabled to “average” to high achieving.)