Autistic Spectrum disorders can show a wide range of symptoms and behaviors, and while there are some specifics that are very common, children shouldn't be diagnosed just based on what looks similar to other children with the disorder. Teachers may have a lot of experience, but they can get used to dealing with certain aspects of the disorder and then think that all kids with those issues may be autistic, but there is much more to a diagnosis than the social/behavioral issues. On the other hand, kids who don't have those familiar issues, may be passed up by teachers, who don't realize that they are struggling with the other problems that a spectrum disorder may cause.
My 4 year old was diagnosed with Pervasive Developmental Disorder, which is an Autistic Spectrum disorder, and some psuedo-professionals might disagree with any autistic diagnosis for him. My boy is incredibly social and outgoing, transitions through different environments and activities with little concern, makes easy eye-contact, is not shy at all (as a matter of fact, everyone is his friend!), he was an easy baby, almost never cried, and had very few discernable sensory issues. On the other hand, he has very low muscle tone, was very late in meeting physical milestones (still has some balance issues with walking, running, etc), only used about 5-10 words by 3 years old, didn't respond to his own name most of the time, and would become completely absorbed in a single action or activity for extremely long periods of time.
We tested his hearing, had neuro-exams for his low activity level, and psycho-educational exams, as well as a Regional Center evaluation. We were shocked by the diagnosis because we thought that all autistic kids were non-social, had difficulty with transitions, and had behavioral issues. Our's did not fit this model at all. I had even worked with autistic kids as a teacher assistant, and thought I knew what autism looked like.
I am glad that we went with the recommendations of the doctor and the Regional Center. I had spoken to teachers who didn't think that Oliver's symptoms and behaviors sounded like autistic spectrum. I didn't even think that they did. Since he started receiving services through early intervention with Regional Center followed by the school district about 18 months ago, he has incresed his vocabulary to being able to use 2-3 word sentences and has become physically stronger and more stable with the work of a devoted adapted physical education teacher. His social skills, which we thought were just fine all along, are being honed through learning to listen, follow commands, and to respond. We are extremely pleased that he is doing so well, and have much hope that he will be able to catch up enough to mainstream in about a year and join his peers in Kindergarten, with some extra assistance.
I would recommend that you let the teacher know that you have had medical and regional center evaluations and that your son, thankfully, does not have to overcome the difficulties of Aspberger's. (in other words, please stop trying to diagnose my child, the appropriate professionals have done that) Now, you can work together to create a system that will be the least traumatic way to help him deal with the behaviors that may be causing him some frustrations in social situations. Maybe you can see if your school has a resource specialist who can make recommendations. Also, there are some great books to teach parents about helping children overcome shyness and be more proactive in taking care of their own needs. You don't need a diagnosis to give your son some help with social skills.