Death at the opera

From Goodreads: "Hillmaston School has chosen The Mikado for their next school performance and, in recognition of her generous offer to finance the production, their meek and self-effacing arithmetic mistress is offered a key role. But when she disappears mid-way through the opening night performance and is later found dead, unconventional psychoanalyst Mrs Bradley is called in to investigate. To her surprise she soon discovers that the hapless teacher had quite a number of enemies - all with a motive for murder."