The strong association of educational attainment with health and mortality is well documented, but whether and how much of this association is causal remains less well understood. In this paper, we examine the impact of education on a range of biological markers representing cardiometabolic, immune, and liver and kidney health. We implemented a Regression Discontinuity Design (RDD) to assess the causal effect of a French compulsory schooling reform, which raised the minimum school leaving age from 14 to 16 for all children born after the 1st of January 1953 on these biological markers of health in middle age. In conventional linear regression models, we find a clear educational gradient in biological risk, with respondents with secondary or tertiary education having significantly lower risk profiles than those who only have primary education. RDD analyses indicated that the reform increased average school leaving age by almost three months, with this increase exclusively driven by participants from blue collar families. Contrary to expectations, we found that eligibility for the reform was associated with higher levels of biological risk, particularly diastolic blood pressure and white cells count. These results are robust to sensitivity analyses and may suggest increased scrutiny of how policies to increase education impact health, and for whom.