“I’m a Liberal Professor, and My Liberal Students Terrify Me,” read the headline of an essay for the liberal website Vox earlier this year. The author, who was frightened enough to write under a pseudonym, admitted that he “cut out anything I could see upsetting a coddled undergrad,” including books by Mark Twain.

The American Association of University Professors last year warned: “The presumption that students need to be protected rather than challenged in a classroom is at once infantilizing and anti-intellectual.”

The...