We identify the effect of individual high school principals on graduation rates and English exam scores using an administrative data set of grade 12 students in BC Canada. Many principals were rotated across schools by districts, permitting isolation of the effect of principals from the effect of schools. We estimate the variance of the idiosyncratic effect of principals on student outcomes using a semi-parametric technique assuming the effect is time invariant. We also allow for the possibility that principals take time to realize their full effect at a school. (Contains 2 figures and 11 tables.)