I get this question all the time.  Here’s a list of the all-girls independent schools, ranked, in alphabetical order:

Brearley School

Chapin School

Convent of the Sacred Heart School

Hewitt School

Marymount School

Nightingale-Bamford School

Spence School

For a directory, look at: NYC Private Girls Schools Contact Info  Note that Marymount and Sacred Heart are Catholic independent schools, the rest are secular.

All the Manhattan girls schools continue through high school.