The current study sought to investigate the agreement between teachers and parents about the mental health of first-grade students, the factors that affected this agreement and the associations between measures completed by students, parents and teachers. The investigation used baseline data collected during the PAX Good Behavior Game (PAX GBG) effectiveness study in 42 Estonian schools (N = 708). Information was collected about externalizing and internalizing difficulties and prosocial behaviour, and about inhibitory control. Our research showed higher agreement between parents and teachers about externalizing behaviour and lower agreement regarding prosocial behavior and emotional problems. Inhibition was correlated with teacher-rated questionnaires, but not with parents' responses. Sociodemographic factors influenced the agreement between teachers and parents somewhat differently. This study highlights the importance of a multi-informant approach in students' mental health assessments, as some problems might be less observable in certain environments or by some respondents. The practical implications of these findings are discussed and suggestions are provided for the development of a school-based mental health screening system.