Existing studies on single-sex schooling suffer from biases due to student selection to schools and single-sex schools being better in unmeasured ways. In Trinidad and Tobago students are assigned to secondary schools based on an algorithm allowing one to address self-selection bias and cleanly estimate an upper-bound single-sex school effect. The upper-bound effects show that while students (particularly females) with strong expressed preferences for single-sex schools may benefit from attending them, most students perform no better at single sex schools. I show that the treatment effect for the typical single-sex student differs greatly from that of the average student. Girls at single-sex-schools take fewer sciences courses and more traditionally female subjects.