In a study conducted across an urban school district, we tested a classroom-based intervention in which students were taught online evaluation strategies drawn from research with professional fact checkers. Students practiced the heuristic of "lateral reading": leaving an unfamiliar website to search the open Web before investing attention in the site at hand. Professional development was provided to high school teachers who then implemented six 50-minute lessons in a district-mandated government course. Using a matched control design, students in treatment classrooms (n = 271) were compared to peers (n = 228) in regular classrooms. A multilevel linear mixed model showed that students in experimental classrooms grew significantly in their ability to judge the credibility of digital content. These findings inform efforts to prepare young people to make wise decisions about the information that darts across their screens.