I don't believe there is a "legal qualification" to be considered an expert witness. The main concerns would be training and experience. Typically, a social worker does not have the necessary training or experience in diagnosing learning disabilities to be considered an "expert". A special education teacher may or may not have sufficient training and experience to be considered an expert. In general, a school psychologist would be the most likely "expert witness" in such a situation because their license assures a certain level of "expertise" pertaining to LD diagnosis.