Federal law allows service dogs to go where ever they are needed in public — including to school. Think of what a dog can do for any child: give unconditional love, exemplify loyalty, faithfulness, courage. For autistic children they can offer a special bond and assist the child to become more social.
Check out what is necessary for your dog to be certified as a service dog. For any person with disabilities a service dog could assist in important ways. And it is simply right that the dogs be able to assist in public places including schools.