The Supreme Court ruled Friday that schools violate religious liberty when they bar parents from opting their children out of classroom instruction involving LGBT-themed books, a major win for religious conservatives.

Justices voted 6-3 along ideological lines to back a group of religious parents affiliated with Montgomery County Public Schools in Maryland who argued they should be able to opt their children out of lessons with LGBT themes.

Justice Samuel Alito penned the opinion, writing that the school district burdened the parents’ First Amendment rights.

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“Today, we hold that the parents have shown that they are entitled to a preliminary injunction. A government burdens the religious exercise of parents when it requires them to submit their children to instruction that poses ‘a very real threat of undermining’ the religious beliefs and practices that the parents wish to instill,” Alito wrote.

Parental rights advocates and religious groups celebrated Friday’s decision.

“Storybooks like those at issue in Mahmoud convey normative messages on sex and gender, and those messages can and often do conflict with the beliefs of parents and their children,” said Sarah Parshall Perry, vice president and legal fellow at Defending Education.

“Today’s ruling should act as a directive to every American school district: notify parents of controversial material, and provide them the opportunity to opt their children out — especially if you provide opt-outs for other curriculum or other reasons,” Perry said.

Kelsey Reinhardt, president and CEO of CatholicVote said the case was a “major victory” but that it should “alarm every American” that the high court had to step in.