(CNN) An elementary school art lesson meant to encourage students to explore activism resulted in a First Amendment violation when the teacher censored students' Black Lives Matter posters, the ACLU claims.

A volunteer teaching a lesson on art and activism at Sacramento's Del Paso Manor Elementary School in September asked students to create a poster focusing on change they wanted to see in the school, according to statements from both San Juan Unified School District, which includes the school, and the American Civil Liberties Union of Northern California.

The problem began when some students who created Black Lives Matter posters were told they needed to redo the assignment and didn't have their work displayed in the classroom, the ACLU says. In a letter sent Thursday, the group called out the school district for censorship, claiming the school is violating the First Amendment.

What is appropriate and "inappropriate"

The ACLU claims the teacher specifically said posters relating to Black Lives Matter were "inappropriate for the class" and made four students who supported the movement in their work redo their posters. The teacher targeted these students based on the content of their poster, the ACLU says, which is therefore a violation of their First Amendment rights, according to the watchdog group.

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