A recent study suggests that parent report on the Social Symptom and Prosocial scales of the Childhood Joint Attention Rating Scale provides useful information about differences in the social development of school-aged autistic children. The current study provides additional psychometric data on the Childhood Joint Attention Rating Scale regarding the longitudinal stability of its scales, its construct validity, and its sensitivity to differences in the social development of clinical samples of children. The study included 64 autistic children without co-occurring intellectual disability, 27 children with symptoms of attention deficit/hyperactivity disorder, and 36 neurotypical children between the ages of 10 and 18 years. Results indicated that scores from parent report on the Childhood Joint Attention Rating Scale were stable across a 15-month period in middle childhood for the three groups and groups received significantly different Childhood Joint Attention Rating Scale scores. Finally, construct validity was supported by the observation of correlations between tester observations of items on Autism Diagnostic Observation Scale-2 Social Affect and subsequent parent report on the Childhood Joint Attention Rating Scale. These results provide further evidence that the Childhood Joint Attention Rating Scale provides meaningful and potentially unique information about prosocial and social symptom development of school-aged autistic children.