This article reports on a subset of results from a larger study which examined middle and high school students' probabilistic reasoning. Students in grades 5, 7, 9, and 11 at a boys' school (n = 173) completed a Probability Inventory, which required students to answer and justify their responses to ten items. Supplemental clinical interviews were conducted with 33 of the students. This article describes students' specific reasoning strategies to a task familiar from the literature (Tversky and Kahneman, 1973). The results call into question the dominance of the availability heuristic among school students and present other frameworks of student reasoning. (Contains 2 tables.)