A three-judge panel on the Ninth Circuit reversed a district judge's ruling Wednesday, the Los Angeles Times reported.
The court said that there is no "heckler's veto" on free speech, that the right can not be limited by the reaction of the audience.
The Center for Bio-Ethical Reform filed suit against the Los Angeles County Sheriff's Department after the 2003 incident at a middle school in Rancho Palos Verdes. Assistant Principal Art Roberts called the department because a truck with pictures of fetuses on three sides was circling the school. Roberts testified at the lower court hearing that he had to stop boys from throwing stones at the truck and that the pictures upset many students.
Deputies searched the truck, questioned the activists for more than an hour and then ordered them to leave the area of the school.
"It's the off-putting speech that needs protection, otherwise there is no need for the First Amendment," said Robert Muise, a lawyer for the center.
Pregnant Mila Kunis wins 'Best Villain' at MTV Movie Awards
Biologists detail four new deep-sea 'killer sponges'