As enrollments in CS1 courses continue to rise, it has become essential for CS educators to support students with varying learning needs and prior programming experiences. Many experts have pointed to the use of mastery-based learning (MBL) which allows students to develop proficiency by engaging in formative practice problems at their own pace. However, less is known about the characteristics of students who use and benefit from such an approach. CS educators need strong evidence for whether formative practice helps to increase aggregate learning outcomes, especially among students who could gain the most from MBL. In this paper, we are interested in exploring the characteristics of students who engage with formative learning opportunities. We analyze data from 118 students enrolled in a CS1 course who were provided with weekly optional practice quizzes that contained multiple choice and free response questions. We used logistic regression to analyze who actually attempted these optional quizzes and found that while gender was not significant, students who do not have prior programming experience (PPE) were more likely to use optional practice than those with PPE. We also conducted a nonparametric two-sample analysis and found that students without PPE engage with optional practice questions to a higher level than students with PPE. Our findings explore the factors that may underpin students’ agency and their academic behavior and performance. These results can inform educators on how to scaffold students’ learning trajectories by accounting for expected group-based behavioral patterns while utilizing MBL in large CS1 courses.