Sorority Girl by Anne Emery (1952)

"I hereby pledge myself not to become part of any secret society at Sherwood High School."  Signed, Jean Burnaby.

But the Nightingales were different -- a service auxiliary.  So they said.

"Frankly, I think you'd better let it alone," said her mother.

"This is something every girl in school wants to be," Jean said, "every single girl!  Do you realize they take only about twenty or twenty-five girls a year? And they're choosing me!"

Another story about the Burnaby family.  (from the back cover)