<p>We estimate the effect of school funding for disadvantaged students on academic underachievement in primary education. For this purpose, we exploit a policy in Belgium that granted additional funding for schools with a share of disadvantaged students of at least 10%. Combining stochastic frontier analysis (a method to measure efficiency) with regression discontinuity design, we find that while additional funding does not affect achievement, it does help reduce underachievement. As a result, the policy has helped bringing students impacted by it closer to realizing their full potential compared to their peers who were not included in the policy. From a policy perspective, our findings indicate that maintaining targeted remedial measures is essential for enhancing effectiveness. Additionally, we emphasize the importance of analyzing both underachievement and achievement.</p>