Schools and teachers have an obligation to uphold the existing and long-established Laws and Professional/Ethical Codes that govern K12 education. Likewise, schools and individual teachers, have a moral and ethical responsibility to engage in full disclosure and truth-in-advertising in the conduct of their professional (teaching) affairs and parents have a legal right and parental duty to insist on this.
These codes call for neutrality, impartiality, and balance when dealing with matters of contention, and for teachers to stay within their areas of certification/training in the classroom. Teachers or schools with hidden agendas or ideological commitments beyond providing high-quality education have a moral and ethical duty to operate in good faith and disclose them, so that parents (who have legal authority over the education of their children) can make fully-informed decisions about how best to educate them.
Relevant court cases:
Several court cases recognize parental legal authority. See, for example:
You can learn more about parental rights in K-12 education at these sites:
Minersville School District v. Gobitis (1940)