I’ve been wondering about this. Jeannie Suk in the New Yorker on “the looming Title IX crisis”:
… Read the rest…on May 13th the Education Department’s Office for Civil Rights (O.C.R.) and the Justice Department’s Civil Rights Division issued a Dear Colleague letter announcing to the nation’s schools that, under Title IX—the 1972 law banning sex discrimination by schools that receive federal funding—transgender students must be allowed to use rest rooms that are “consistent with their gender identity.”…
In chastising North Carolina, the Justice Department explained that if non-transgender people may use bathrooms consistent with their gender identity, then denying transgender people access consistent with their gender identity constitutes discrimination on the basis of sex. Similarly, the Dear Colleague letter states that