The Baby-Sitters Club #12: Claudia and the New Girl book by Ann M. Martin

Claudia has always been the most outrageous girl in her class...until Ashley Wyeth comes along. Ashey's really different - she dresses in hippie clothes, wears six earrings, and is the most fantastic artist Claudia has ever met.
Ashey says Claudia has artistic talent, too. She thinks Claudia should spend more time on her "calling" and forget about the Baby-sitter's Club: It's just a waste of time.

The Baby-sitters are sick of Ashley Wyeth, and they think Claudia's a traitor. Claudia's got to decide: either the Baby-sitter's Club or the new girl - one of them's got to know.