The image of Superman promoting tolerance and diversity among schoolchildren is authentic. It's a 1949 image by comic book artist Wayne Boring, used in school posters and book covers. It's a relic of a campaign to stamp out racial, ethnic, and religious prejudice in the United States at a time when Europe was undergoing a seismic swell of anti-semitism in the path to World War II.
In 1938, the New York City Board of Education began requiring students to learn about how multiple groups contributed to American history. When World War II erupted one year later, the demand for tolerance education spiked. The New York Times reported in 1939 that "Instances were cited of teachers in New York City and elsewhere being 'ridiculed, harassed and otherwise impeded' by pupils under the influence of, and stimulated by, Nazi doctrine." To nip foreign propaganda in the bud, schools across the country joined the tolerance movement. Military leaders encouraged it, too. They knew that American troops, many of them fresh out of school, would fight their best if they learned to set aside their differences.
Countless non-profit groups, many of them interreligious, led the charge. Burkholder writes that “Religious leaders, educators, and politicians stressed tolerance as a central tenet of democracy." They provided prejudice-fighting materials to schools, from teachers’ manuals to comic books to textbooks.
Outside of school, short pro-tolerance films played at the beginning of movies. People held tolerance rallies. The National Conference of Christians and Jews distributed 10 million “Badge of Tolerance” buttons. Groups such as the Council Against Intolerance in America distributed maps showing the breadth of diversity in America’s cultural landscape. Even Superboy stepped in, telling a bunch of his schoolmates that “No single land, race or nationality can claim this country as its own.” At the end, Superboy and his pals celebrate by eating Swedish meatballs.
That all sounds well and good, but it didn't last. The groups that led the charge for tolerance were eventually accused of being "un-American" themselves! Read what happened at mental_floss.