Murder Crossed (Henry Holt Mystery Series) (1996)

Clara Gamadge, widowed grandmother and amateur sleuth, has returned to her alma mater, Woolcott Academy, at the request of the headmistress, Louise Littleton, her old friend and classmate. It seems another old school chum, now an famous actress, is visiting with her three daughters to enroll them in school while she performs in London for three months. When Margo returns to pick up her children, her body is found stabbed to death on Woolcott's grounds. But is it really Margo? Through a parade of TV cameras and ex-husbands, Clara noses around for the truth.