Background <p>Pupils’ aggressive behaviour towards teachers is widespread. This study examined the prevalence and types of pupils’ aggressive behaviour towards teachers across different grades and the associations between social capital and the risk of pupils’ aggressive behaviour in a one-year follow-up study design.</p> Methods <p>We collected questionnaire data from 2018 to 2019 and 2019–2020. A total of 1198 teachers from 94 schools in Denmark participated in both rounds. The associations were analysed via multilevel logistic regressions.</p> Results <p>Teachers are frequently exposed to pupils’ aggressive behaviour, particularly those teaching the youngest pupils. Being called degrading names was the most frequent type of harassment, being yelled in a threatening manner was the most frequent type of threat, and being hit was the most frequent type of physical violence. Social capital (bonding) at baseline was statistically significantly associated with a reduced risk of pupils’ harassment and threats one year later. Furthermore, social capital (linking) at baseline was statistically significantly associated with a reduced risk of threats each year.</p> Conclusion <p>Social capital can reduce the risk of pupils’ aggressive behaviour in the form of harassment and threats. Therefore, fostering a supportive climate in which teachers can openly discuss pupils’ aggressive behaviour with their colleagues and supervisors is recommended.</p>