Aim: Strong associations have been found between being exposed to self-harm in family and friends and own self-harm in adolescence. Therefore, self-harm and suicide behaviour might tend to cluster within school and school classes. The aim of this study was to describe the prevalence, frequency and type of self-harm, suicide ideation and suicide attempts within Danish high schools and to test whether self-harm and suicide behaviour cluster in schools and school classes. Methods: Data came from the Danish National Youth Study 2014, a national survey. The respective study included 66,284 high-school students nested in 117 schools and 3146 school classes. The prevalence and clustering of self-harm behaviour, ever and within the last year, type of self-harm (e.g. cutting, burning, scratching and hitting) and suicide ideation and suicide attempts were investigated. Multi-level logistic regression was applied to quantify clustering among participants within the same class and school. Results: In total, 12,960 (20%) reported self-harm ever and 5706 (8.6%) within the last year. Prevalence was higher among girls than boys. Among girls, cutting (15%) and scratching (13%) was the most common type of self-harm, whereas among boys, hitting (6.7%) was most prevalent. The degree of clustering of self-harm and suicide behaviour was low, with school-level intra-class correlation coefficients (ICC) ranging from 0.8–1.8% and school class level ICC’s from 4.3–6.8%. Conclusions: This study shows that self-harm was common, especially in girls. The degree of clustering of self-harm and suicide behaviour in school and school classes was low.