The letter identified the teacher only as Mr. Madden; a school directory lists a David Madden as a teacher. Ms. Conner would not identify the parent volunteer, who was referenced in the school district statement only as Ms. Kincaid. A Magali Kincaid is listed as a parent or community member on the school’s website.

Neither Mr. Madden nor Ms. Kincaid could be reached on Sunday. Kent Kern, the school district superintendent, to whom the letter was addressed, also could not be reached.

Ms. Kincaid went to the principal, who supported Mr. Madden, according to the letter. The principal said that Black Lives Matter posters are political statements and therefore were off limits for public display.

Though not named in the letter, Damon Smith is listed on the school’s website as its principal. He could not be reached on Sunday.

The A.C.L.U. argued that the Black Lives Matter posters were protected speech under the California Education Code because they convey a student’s thoughts, ideas and beliefs in the support of black lives. The group also said the posters were protected under the California Constitution.

Ms. Conner wrote that “there are obvious problems with a teacher and principal who currently have black students in their classroom and school taking the positions that the acknowledgment of Black Lives is controversial and political in nature.”

The district has not responded to the letter, Ms. Conner said.

“Because these are so basic fundamental rights we believe that the school district at this point would have responded back to us,” she said on Sunday.