This paper analyzes a questionnaire that investigates teachers' perceptions of working in a full-day experimental program in Croatia. The aim of the research is to analyze the questionnaire that examines teachers' opinions on full-day classes, with a particular emphasis on determining the quality of the measurement instrument. The intention is to verify the structure of the existing questionnaire and improve its validity and reliability. Factor analysis was used in the research to confirm the grouping of items according to expected factors and ensure the contextual validity of the instrument. In addition to factor analysis, descriptive statistics of the teachers' responses are presented. The results showed that certain items from the initial questionnaire had low factor loadings, indicating the need for adaptation or removal. Based on the results, a new questionnaire was created in which specific items were modified to achieve better measurement accuracy. The improved version of the questionnaire enables more reliable measurement of teachers' attitudes towards full-day classes. Descriptive statistics indicated that teachers see potential and benefits in such an organization of classes, but also notice many shortcomings. The analysis of the questionnaire confirmed the division of statements and internal consistency of factors, explained items with low factor loadings, and resulted in a new questionnaire for further research. This research contributes to a better understanding of the structure of teachers' attitudes and serves as a basis for potential future research on full-day classes.