Student engagement is a central construct in education research, as it is a strong predictor and/or mediator of both cognitive and non-cognitive student outcomes. However, there remain questions among researchers about how to properly measure student engagement during lessons. Our study contributes to this discussion by presenting the development and validation of a theoretically grounded questionnaire of students' lesson engagement, which is based on an experience sampling methodology. To validate this questionnaire, we issued it to 410 fifth- and sixth-grade students (10-12 years old). The results of both an exploratory and multilevel confirmatory factor analysis approved construct validity of the measure. Adequate within-person variability across time points was also found, which makes the questionnaire suitable for capturing fluctuations over time. Furthermore, partial scalar measurement invariance for questionnaire administration across time points and between fifth- and sixth-grade students was found.