1. The "parents" are not married. A parent and that parent's children are living with a pervert. Legal recognition of unicorns wouldn't make unicorns real.
2. The school was foolish to put their reason into writing. They should reject the child (given that they decide to reject) without giving a reason - never give a reason, especially in writing.
Certainly not in Tennessee. 81% of the voters passed a constitutional amendment recognizing marriage as only between one man and one woman.
Furthermore, they added, "If another state or foreign jurisdiction issues a license for persons to marry and if such marriage is prohibited in this state by the provisions of this section, then the marriage shall be void and unenforceable in this state."
Good for them.