“We have tremendous empathy for people who are fearful and scared,” Ms. Lieberman said. “We’ve really tried to explain why the schools are safe now.”

Malibu parents have a history of skepticism about official health advice, including routine childhood vaccinations: At some local elementary schools in 2014, fewer than 60 percent of kindergarten students had received the full lineup of recommended vaccines, far below the state average.

In this case, the distrust on both sides became plain last fall, when supporters of caulking removal secretly took their own samples from classrooms and had them independently tested. Ms. deNicola announced that the results showed extraordinarily high levels of PCBs. The school district asked the sheriff’s office to investigate her for trespassing and vandalism.

The battle now threatens to tear apart the school district: Concern over PCBs has fueled an existing effort here to break away from Santa Monica so that Malibu can be in control of its own schools.

Beth Lucas, a parent, pulled her son, Christian, out of Malibu High after their endocrinologist said it was especially dangerous for him to remain there. Christian, now 17, had a malignant brain tumor at age 6, and the radiation used to treat it left him with a diminished immune system and thus more vulnerable to the effects of PCBs, the doctor told the family.

“We moved to Malibu for the schools, so it has been a big slap in the face to have the school district treat the parents and teachers and children so poorly,” Ms. Lucas said. She is also considering removing her daughter, who is in middle school, at the end of the year, but worried about the cost of private school.

“Yes, we live in this nice house,” she said, sitting on a hilltop porch that overlooked a wide expanse of ocean. “I don’t want to have to sell my house and leave Malibu. The district has put us in a horrible position.”