The elusive character of student agency makes it a relevant construct to be investigated and measured. An initial effort in this direction was represented by the Agentic Engagement Scale, a five-item instrument designed to assess the degree to which students constructively contribute to the flow of the instructions they receive from the teacher. Despite its merits, in its current form this scale takes into account only a part of the wide range of student agentic expressions. In the present work, we propose an extension of the Agentic Engagement Scale from five to 10 items. Compared to the original scale, the new version covers a larger variety of proactive student contributions, such as those concerning peer interactions and those communications in which learners question or challenge the teacher's instructions. The study was conducted on 1,064 Italian high school students equally distributed between males and females. Confirmative factor analysis endorsed the adequacy of a one-factor structure of the enlarged Agentic Engagement Scale, which showed good psychometric properties as well as positive associations with student motivation to learn and the other three aspects of engagement (i.e., affective, behavioral, and cognitive). The theoretical and practical implications of a more comprehensive scale of student agentic engagement are discussed.