K.P.
Autism is a form of PDD. Think about what that term actually says... delays that pervade across multiple areas of development.
Teachers who devote their careers to children with Autism are incredible people, but they do not have the training to make an accurate diagnosis. It's not their job and quite frankly... experience is wonderful, but training in assessment and diagnostics is entirely different. I have worked with some very gifted special educators (as a psychologist and now a school administrator) and while they can teach your child to do things you never thought possible, they also tend to "see what they know".
Contact your local Autism Society and ask who they refer families to. In my former life (school psychologist), I consistently referred families to neuropsychologists NOT neurologists b/c neurologists look at physical development and medical symptomology, not behavior and emotionality. A neuropsychologist works with you and your child over a period of 10-15 hours to conduct a comprehensive assessment that includes the neuro info from the doctor but adds in the behavioral and social-emotional factors. Neuropsychologists also typically request your consent to speak with the teacher to gain that insight!