In Tennessee can a 16-year-old choose to go back and live with a parent that has a past history of proven drug usage and where the child was delinquent from school to the point of failing?

Here's the code for CA - Tenessee is probably similar. What are the best interests of the child?

3042. (a) If a child is of sufficient age and capacity to reason so as to form an intelligent preference as to custody, the court shall consider and give due weight to the wishes of the child in making an order granting or modifying custody

A 16-year-old cannot just choose where they want to live. They must abide by the custody order of the court. The non-custodial parent can petition the court to change custody. In that event, the child would be able to advise the judge of where it is that they would prefer to live, but the judge makes the final decision. In the situation you described, *merely in my personal opinion*, it's unlikely the judge would grant this parent custody. However, I am not an attorney, and you might want to speak with one.