This letter should follow a general response that the school can’t permit a child to “opt out”. It asserts that you were not asking permission, informs the school that unless they respect your parental rights your child will not be in attendance, and contains required “statement of assurance” language to establish that you will engage in home schooling during STAAR dates. This should provide a level of protection against truancy threats.
The school may respond by claiming you have to withdraw your child to do this. This is untrue. You can ignore that response. You have stated your position.