In single-sex classes, which are common for sex education, students must be allowed to go to the class of the gender they consider themselves to be rather than their actual gender, the U.S. Department of Education directed schools that receive federal funds in a Dec. 1 memo. The memo, from the department's Office for Civil Rights, provides guidance for K-12 schools for when they are allowed to have same-sex classes or extracurricular activities and remain in compliance with Title IX, a 1972 law to protect against gender discrimination in education programs that get federal aid. Schools may divide students by...