Educational effectiveness research has demonstrated the importance of teacher beliefs and attitudes for student learning. Beliefs strongly determine teachers’ thoughts and actions and they ultimately influence student achievement, even though they may do so indirectly. In order to change people’s practices and behaviours, it is necessary to first change their beliefs. Studies of beliefs and attitudes draw on a variety of concepts. This study explores the relationship between academic optimism and the mathematical achievement of Czech lower secondary students and follows up on previous studies that confirmed the relevance of this concept for the Czech education system. The data were collected as part of work on the Czech Longitudinal Study in Education (CLoSE) in October 2012 and May 2016. The database used in the analyses contains data from 124 basic schools and 39 grammar schools, and from 4798 students and 1469 teachers. Two-level structural equation modelling showed that there is a statistically significant link between the academic optimism of schools and students’ mathematical achievement, even after controlling for prior achievement and socioeconomic status at both the student and the school level. This finding has important implications for pre-service and in-service teacher training and for education policy.