An elementary school art lesson may have resulted in a First Amendment violation when a teacher censored students' Black Lives Matter posters, according to the American Civil Liberties Union of Northern California.

A volunteer at Sacramento's Del Paso Manor Elementary School was teaching a lesson on art and activism in September and asked students to make a poster focusing on change that they wanted to see in the school, according to statements from both San Juan Unified School District and the ACLU.

When some students created posters that involved the Black Lives Matter movement, they were told to redo their posters. The school also did not display their work in the classroom.

The ACLU sent a letter on Thursday to the school, claiming that it violated the First Amendment. The organization says that the Black Lives Matter posters were targeted because of their content.

The district claims that the students were told to redo the assignment because it focused on large social issues, rather than ones specifically related to the school, and therefore did not meet the assignment requirements. A spokesperson for the district told CNN that students whose posters focused on immigration and animal cruelty were also asked to redo the assignment.

The ACLU also claims that the school broke the California Education Code, which says that, "Pupils of the public schools, including charter schools, shall have the right to exercise freedom of speech and of the press including, but not limited to, the use of bulletin boards, the distribution of printed materials or petitions, the wearing of buttons, badges, and other insignia." The organization claims the posters fall under this category.