Disengaged test taking tends to be most prevalent with low-stakes tests. This has led to questions about the validity of aggregated scores from large-scale international assessments such as PISA and TIMSS, as previous research has found a meaningful correlation between the mean engagement and mean performance of countries. The current study, using data from the computer-based version of the PISA-Based Test for Schools, examined the distortive effects of differential engagement on aggregated school-level scores. The results showed that, although there was considerable differential engagement among schools, the school means were highly stable due to two factors. First, any distortive effects of disengagement in a school were diluted by a high proportion of the students exhibiting no non-effortful behavior. Second, and most interestingly, disengagement produced both positive and negative distortion of individual student scores, which tended to cancel out much of the net distortive effect on the school's mean.