Over the past three decades, many countries have introduced iodized salt policies to eradicate iodine deficiency. Iodine deficiency in utero is detrimental to cognitive ability, but little is known about the consequences of iodine deficiencies after birth. This paper examines the impact of iodine deficiency in adolescence on school performance. I exploit the introduction of iodized salt in Denmark during 1998–2001 as a natural experiment. Combining administrative records on high school grades over a 30-year period with geographic variation in initial iodine deficiency, I find that salt iodization increases the grade point average of students by 6–9 percent of a standard deviation.