Single-sex public schools in the USA

I didn't know this:

In 1995, only three public schools in the United States offered single-sex educational opportunities. As of November 2006, at least 253 public schools in the United States are offering gender-separate educational opportunities. Most of those are COED schools which offer single-sex CLASSROOMS, retaining at least some coed activities (in some cases, only lunch and certain electives are coed). However, 51 of those 253 schools are COMPLETELY single-sex in format.

My kids' school just has single-sex sex-ed classes.