For what it's worth, Chaminade HS was/is a very large all-boys college-prep Catholic high school there in Long Island. (I'm hedging on the tense -- "was/is" -- because, like other such institutions, CHS may have become coeducational in the years since I went to college with a few of its graduates.)

Ergo, the three girls who joined The Glencoves could not have been schoolmates, and membership may likely have increased from 3 to 6 sometime after the group's high-school beginnings.