Flexibility is an important element in learning mathematics. The purpose of this study was to investigate whether flexibility in linear equation solving predicts future academic achievement in mathematics and other subjects. Participants were 149 Finnish high-school students. Results show that flexibility was related to grades in both tracks of mathematics, chemistry, and mother tongue, as well as the total number of exams taken in the national matriculation examination. However, when controlling for accuracy in equation solving, only basic level mathematics and, to some degree, chemistry grade were related to flexibility. On the other hand, flexibility had an impact on students' choice to participate in the mathematics and physics exams. A theoretical analysis shows that student selection may mask part of the relationship between flexibility and grade.