Background: Knowledge about oral health becomes essential, especially during school years when the prevalence of dental problems is high. The objective was to assess whether an educational intervention improves teachers' knowledge of oral health. Methods: The sample was obtained through a census of elementary school teachers from public schools in Brazil. A structured questionnaire, consisting of questions about demographic data, knowledge of dental caries and periodontal disease, oral hygiene practices, preventive dentistry, and attitudes toward oral health, was administered three times: before, immediately after, and three months after the educational intervention. An educational intervention was conducted through a face-to-face session (1-h lecture) in all schools, addressing oral health topics (aforementioned). Descriptive analysis and comparison tests (McNemar and Wilcoxon) were employed to assess the impact of the intervention on teachers' knowledge. Results: Eighty-three teachers agreed to participate (89% adherence rate), and 53 teachers completed all three questionnaires (64% final retention rate). The results showed that the intervention improved teachers' knowledge immediately after and three months after the intervention (p < 0.05) in responses related to knowledge of dental caries, periodontal disease, oral hygiene practices, and preventive dentistry. Conclusions: This study demonstrated that teachers' knowledge of oral health improved after the intervention.