Purpose School students are at risk of unintentional injuries. Teachers are the main caregivers and the first protectors for school students and must be able to deliver first aid. This study aimed to assess the school teachers’ knowledge about first aid in Kashan, Iran. Methods A cross-sectional study was undertaken on a sample of 200 teachers working in public and private primary, secondary and high schools in Kashan city, Iran. Data collection was achieved via a questionnaire consisting of items on teachers' characteristics and 20 questions on basic first-aid practices. One point was given for each correct answer. The lowest and highest possible scores were 0 and 20. Then, the sum scores 0–9, 10–16, and 17–20 were ranked as weak, moderate, and good first aid knowledge, respectively. Statistical analyses were performed using descriptive statistics, Chi-square and Fisher's exact tests, and regression analysis. Results The mean age and working experience of teachers were respectively (41.37 ± 6.35) years and (20.07 ± 7.29) years. Among all teachers 51.0% were males and 66.3% were working in public schools. Totally, 40.3% of the teachers had passed courses on first aid and 48.5% had confronted with situations needing first-aid; 59.7% evaluated their own knowledge of first aid as moderate. The mean score of first-aid knowledge was 10.00 ± 2.58. Teachers who had received first aid training and those with previous experiences of situations requiring first-aid possessed higher scores ( p < 0.001). Most teachers answered wrongly to the questions regarding first aid in a child with a fracture (93.4%) or struck by electricity (85.2%). Conclusion Most of the teachers had insufficient knowledge on first aid. They mostly did not pass any training on first aid. Urgent action seems necessary for training teachers on first aid through in-service training programs.