Although reflection is a key behaviour of expert designers, it is often a challenging task for new designers. In addition, research on the reflectivity of student designers is limited. The purpose of this study is twofold: (1) to identify the levels of reflectivity while designing, and (2) to study the relationship between reflectivity and conceptions of informed design. We collected data from high school students engaged in an engineering design project. We developed a coding protocol to score levels of reflectivity in student reflections at three levels (low, medium, and high), and used the "conceptions of design test" to assess changes in student understanding of design activities. Using Wilcoxon signed-rank tests, we determined if students tended to select more 'key' design activities and fewer 'distractors' within each reflection group. We also performed McNemar's tests to determine which specific design activities were important within each reflection group after the design project. The results show moderately reflective students had higher gains in understanding of informed design activities compared to those with high or low reflectivity. Results also indicate that different design activities became important for students within each of the three reflective groups. Implications from this research indicate that groups of students experience changing conceptions of design in different ways. An understanding of what students deem important while designing would better allow teachers to encourage behaviours that are like those of informed designers.