Callas studied with Trivella for two years before her mother secured another audition at the Athens Conservatoire with de Hidalgo. Callas auditioned with "Ocean, Thou Mighty Monster" from Weber's "Oberon". De Hidalgo recalled hearing "tempestuous, extravagant cascades of sounds, as yet uncontrolled but full of drama and emotion". She agreed to take her as a pupil immediately, but Callas's mother asked de Hidalgo to wait for a year, as Callas would be graduating from the National Conservatoire and could begin working. On April 2, 1939, Callas undertook the part of Santuzza in a student production of Mascagni's "Cavalleria rusticana" at the Greek National Opera at the Olympia Theatre, and in the fall of the same year she enrolled at the Athens Conservatoire in Elvira de Hidalgo's class.