The present study aims to expand the current understanding of engagement by examining variations in students' situation-specific engagement in lower secondary school. In addition, the validity and reliability of a new situation-specific InSitu Instrument were examined. The sample consisted of 1809 Finnish students attending Grade 7. The students filled in mobile ratings on their lesson-specific engagement after lessons. Furthermore, they answered questionnaires concerning their overall engagement, achievement beliefs, and task values in math and literacy. The results showed substantial variation within and between students in situational engagement. A five-factor structure was identified for the InSitu ratings: 1) behavioral engagement, 2) emotional engagement, 3) competence experiences, 4) disaffection, and 5) help seeking. Correlational analyses showed moderate to high associations between situation-specific engagement and student engagement, achievement beliefs, and task values. The findings provide support for the InSitu Instrument as a valid and reliable tool for investigating situation-specific engagement in the classroom.