President Obama did not 'mandate' anything, the administration - specifically the Departments of Education and Justice - issued a letter recommending that public schools should permit transgender students to use the restroom which matches their gender. There is the implicit threat of loss of funding, but those who are decrying this move by the President - which amounts to recommending how to execute a policy - as unconstitutional are being laughably hyperbolic.
Given the current generation of Republicans/conservatives and their rocky relationship with Barry, I'd have been surprised if they
didn't cry about this. They keep mistaking modern day America for an America that only exists in a far-right conservative alternate reality.
