The American Civil Liberties Union is threatening legal action against as many as a dozen school districts from Maine to Mississippi unless they stop programs the group says illegally segregate boys and girls into single-sex classes and promote stereotypes, Reuters reports. The group also was demanding that Florida’s Department of Education launch an investigation into widespread single-sex teaching in that state, where 32 schools in 16 districts offer single-gender classes. A spokeswoman for the department said they had not yet received the demand, which is posted on the ACLU’s website. Single-sex education has expanded into as many as 300 public schools in recent years — helped in part by a 2006 decision by the U.S. Department of Education that relaxed restrictions on the practice. That decision, under President George W. Bush, allowed schools to offer voluntary single-sex classes so long as programs did not violate Title IX, a federal law that outlawed gender discrimination in education…