Grades provide students with information about their level of performance. However, grades may also make students more aware of how well they have estimated their performance, their so-called calibration accuracy. This longitudinal quasi-experimental study, set in secondary education, examined how to increase students' awareness of the accuracy of their grade estimates in order to improve their calibration accuracy. During an entire school year, students from year 1, 2, and 3 provided grade estimates after each of their French exams. Subsequently, when students received their grades, the level of reflection support on their earlier estimates was manipulated. The first group of students just received their grade, the second group had to calculate the difference between their estimate and the actual grade, and the third group also had to reflect on reasons for a possible mismatch. We expected that more reflection support would lead to more improvement in calibration accuracy. Results showed that providing grade estimates already improved calibration accuracy over the school year, regardless of level of reflection support. This finding shows that asking for grade estimates is an easy-to-implement way to improve calibration accuracy of students in secondary education.