Can a parent legally override a team physician's decision to not allow a child to return to sports?

No: The team physician is a representative of the school district and follows their guidelines.It is a privilege to play on a school team and not a right. The point though is answering the question as to why you would let them play if the doctor thinkes there is a problem! why risk their health?

At your own risk: Such behavior could easily be considered medical neglect and subject the parent to a visit from child and family services. If the parent were to hide the information from the team officials or school, they could be held legally responsible for any injury or other negative event associated with their choice.Talk with an attorney.