But the authors might argue that its secular purpose is in advancing science education. This argument is thoroughly unsound, of course, because that bill would significantly impair science education, but having a poor secular purpose is not unconstitutional.
But see
Edwards v.Aguillard:
While the Court is normally deferential to a State's articulation of a secular purpose, it is required that the statement of such purpose be sincere, and not a sham. See Wallace v. Jaffree, 472 U.S. at 64 (POWELL, J., concurring); id. at 75 (O'CONNOR, J., concurring in judgment); Stone v. Graham, supra, at 41; Abington School Dist. v. Schempp, 374 U.S. at 223-224.
The fact that creationists can lie about their motives, which they will, doesn't mean that the court has to take their lies at face value, which it won't if their previous failures are anything to go by.