An elite charter school in California has been rocked by scandal since the end of the last school year, when a kindergarten teacher read her class a pair of books advocating transgender ideology, and a male kindergartener was reintroduced to the class as a girl.

Now first-graders at Rocklin Academy Gateway risk a trip to the principal’s office if they refer to their transgender classmate by the wrong name or gender pronoun, said Karen England, executive director of the Capitol Resource Institute, a pro-family group based in Sacramento.

“There was a little girl who had been in class with the little boy all last year,” Ms. England said. “They’re in different classes now, but she saw him on the playground yesterday and called him by his name. The little girl was told ‘you can’t do that, his name is this name,’ and ‘you need to call him a “her.”’ Then she was called to the principal’s office.”

Ms. England said she has been in touch with the girl’s parents and described them as “outraged.” She said they met with school officials to discuss the matter Wednesday.

It’s not clear whether the girl was disciplined for the infraction. Rocklin Academy did not return a request for comment before press time.

The controversy at Rocklin is just the latest example of the transgender movement’s increasing emphasis on spreading its ideology through early education.

In Washington state, for instance, the Office of the Superintendent of Public Instruction issued guidelines instructing schools to teach students about gender identity as early as kindergarten.

Kindergarteners and first-graders must be taught that “there are many ways to express gender,” according to the guidelines, which took effect this school year.

Second-graders, meanwhile, are expected to learn that there is a “wide range of gender roles and expressions,” and third-graders that “gender roles can vary considerably.”

Chris Plante, policy director of the Family Policy Institute of Washington, said expecting 5-year-olds to understand the concept of gender identity is “completely unreasonable and irresponsible.”

“We don’t allow our kids to cross the street unattended at this age because they can’t understand the concept of a car, never mind gender expression,” Mr. Plante said. “That doesn’t even begin to address the idea that these are concepts that should be left to parents, who know best for their children.”

Two days before summer break, a kindergarten teacher at Rocklin Academy read her class “I am Jazz,” written by transgender reality TV star Jazz Jennings, and “Red: A Crayon’s Story,” a children’s book about a blue crayon that identifies as a red crayon.

“The boy’s parents asked the teacher to read these books, that today was going to be the day that we are going to change his name and start presenting him as a girl,” Ms. England said. “And the school said yes.”

At some point the boy reportedly left the classroom and came back dressed in girls’ clothing, whereupon he was reintroduced to his classmates as a girl.

Neither Rocklin Academy nor the teacher involved has confirmed that the boy left the classroom to change.

“There’s some question about the sequence of events because the school is refusing to answer questions,” Ms. England said. “They’re telling parents to ask their 5-year-olds what happened. We interviewed about one-third of the students — again, these are kindergarteners — but they all agreed that he came to school dressed one way and, at some point in the day, changed, and that his name became a girl’s name.”

When they went home, students told their parents that one of their classmates had changed from a boy to a girl.

“All I heard was my son just mentioned his friend, who was a boy, is now a girl,” one parent, Chris Hurley, told Fox40.

About a week later Rocklin Academy sent a letter to parents to reassure them that the books read were “age appropriate children’s books, geared for ages 4-8.” The school pointed to its policy and California state law prohibiting discrimination on the basis of gender identity.

Legal counsel for the Rocklin Academy school board also delivered a PowerPoint presentation to parents on July 31. One slide said students have the “right to the use of his or her preferred pronoun at school and in class.”

“Intentional use of the non-preferred pronoun is considered gender identity harassment,” the presentation said. “Don’t assume a preferred pronoun; ask.”

The presentation cited Obama administration guidance and Title IX, saying the cost of a federal investigation would be “astronomical, in both the financial and reputational senses.”

Rocklin Academy sent parents follow-up letters addressing their complaints on Aug. 11 and 15.

The Aug. 11 letter said the board would not “ban specific books from being read to students in the classroom.” The Aug. 15 letter raised concerns about the “possibility of creating a slippery slope about what can and cannot be discussed in our classrooms.”

Ms. England called the arguments being advanced by the school “disingenuous.” She said parents aren’t asking for specific books to be banned or certain subjects to be off limits for discussion.

“All the parents want is notification before these controversial issues are brought up in the classroom,” she said.

The PowerPoint presentation noted that parents have a right to opt their children out of sex education under California law. But it said “diversity and tolerance curricula” do not qualify as “sex education.”

The Rocklin Academy Board of Directors held a meeting Monday, during which parents were able to vent their concerns. The board, however, refrained from responding because the topic was not on the agenda. A school board meeting to address the issue is expected to be held on Sept. 18.

Classes resumed at Rocklin Academy last week, and it didn’t take long for one of the transgender boy’s first-grade peers to call him the wrong name and use the wrong gender pronoun.

“Imagine how difficult it is for that first-grader to try to understand that the person that she knew as a boy all last year is suddenly a girl,” Mr. Plante said. “And to hold her to account for that, to send her to the principal’s office because she honestly doesn’t understand what this means? It’s mind-boggling.”