A federal judge on Wednesday ruled in favor of 12 Virginia families  who argued that policies making masks optional in classrooms violated the rights of their children, who have health conditions that make them vulnerable to the virus. The case is only the latest example of the legislative back and forth that is accompanying the adjustment to new, post-pandemic norms.

The families filed suit against Gov. Glenn Youngkin, a Republican, and other state officials last month after the governor signed a bill that effectively bars mask mandates in schools.

The law, which went into effect on March 1, gives parents the right to exempt their children from mask-wearing without stating a reason. It codified a step that Mr. Youngkin took with an executive order on his first day in office in January, making mask-wearing optional in schools.

The Virginia Educators Association, which represents more than 40,000 teachers and other school employees, criticized the bill in February, saying that it “would remove local decision-making power from local school divisions and place it into the hands of the state.” Many other states had lifted universal mask mandates but left decisions about keeping the requirement in classrooms up to individual school districts.

On Wednesday, U.S. District Judge Norman K. Moon granted a preliminary injunction to allow the 12 families in the lawsuit to request mask requirements at their children’s schools if recommended by a doctor as a “reasonable modification” while their case proceeds. In effect, the decision challenges the interference of state law with federal disability rights law when a child requires community masking in order to participate in a classroom setting.

The students have health conditions — which include cystic fibrosis, asthma, Down syndrome, lung conditions and weakened immune systems — that make them particularly vulnerable to coronavirus infections, their parents outlined in the complaint.

The judge made clear that families of other vulnerable children will have to pursue their own suits, as the executive order and the state law remain in effect.

“This is not a class action, and the twelve plaintiffs in this case have no legal right to ask the court to deviate from that state law in any schools in Virginia (much less school districts) their children do not attend, or indeed even those areas of their schools in which Plaintiffs’ children do not frequent,” Mr. Moon wrote in the decision.

Eden Heilman, legal director for the A.C.L.U. of Virginia, said that while the injunction is limited to these 12 students, it is “clearly a blueprint” for any parent of a student with a disability who may need classroom masking as an accommodation under the Americans with Disabilities or Rehabilitation acts — which make clear that children with disabilities should not be segregated or denied access to the same education as their non-disabled peers.

“It’s a critical decision and we were thrilled with the outcome for that very reason,” said Ms. Heilman. “Young individuals with disabilities get often overlooked in this debate about whether to mask and parental freedom — and nobody considers the repercussion for these kids.”