This talk illustrates the benefits of using estimated and documented 2nd and 3rd year school grades for content validation of a temperament test and for the investigations of school grades - temperament associations. The results of our studies using Structure of Temperament Questionnaire (STQ-77-C3) showed that estimated and documented grades supported the earlier findings of positive associations between the grades and temperament traits related to sustained attention and intelligence. However, our results showed a much more differentiated pattern of temperament-grades associations. Contrarily to expectations, the Satisfaction scale measuring emotional valence showed no significant (at p <0.01) effects in both samples. The Neuroticism scale measuring dispositional behavioural alertness and avoidance of uncertainty negatively correlated with the grades in Mathematics and Science/Environment in both samples but not with other school subjects. Social-verbal Tempo scale had high correlations with documented grades across all subjects, whereas Social Endurance had a significant correlation only with Art in the sample that used documented grades. There was also a differential pattern of temperament-grades associations for Sensation Seeking versus Probabilistic Processing scales. Our results, therefore, highlight the benefits of using the school grades for content validation of the STQ-77-C3.