Be aware of school policies before inappropriate material gets into classrooms, or your child's hands. Once bad policies and curricula get in, it's often difficult to get them out. Educate school personnel and elected officials on the law and your parental rights.
Ultimately, you will probably not be able to shield your child completely from the topic of same-sex marriage. Even if your child's school does not implement materials on this topic, homosexuality and other controversial sexual themes are frequently featured on TV and in other facets of popular culture.
However, you can prepare your child through intentional interaction with him or her about these topics. It will be especially important to lay a biblical foundation for their understanding of sexuality - that God's design for human sexuality is for it to be expressed within marriage between a man and a woman. By clearly sharing your family's beliefs ahead of time, your child may be better equipped to process material promoting same-sex marriage (or other sexual messages) – as well as discuss those controversial themes with you when they encounter them.