“The Conservatoire is still that gloomy and dirty place that we knew in our day,” wrote Debussy to André Caplet in 1909. Although inaugurated in 1801, it was not until 1881 that plans for the renovation of the building had begun, and it would be thirty more years before the project was completed. However, a ten-year-old boy would not initially notice these conditions; having had as yet no formal academic experience, he was certainly impressed by the high walls, the discipline, and the atmosphere of the historic institution on the rue Bergère. Achille would spend more than ten years of his life there—years that would strongly weigh on his development, no matter what he might say later. It thus behooves us to follow, year by year, not only the prizes he was awarded and the assessments he received from his teachers but also the friendships that were forged, and to attempt to reconstruct the context in which he lived.

In 1872, when Achille entered the Conservatoire, Ambroise Thomas had served as the director of the institution for only one year. After Daniel Auber's long reign, the building needed to be remodeled; a committee had been created in 1870, during the last year of the Second Empire, but its work had been interrupted by the Franco-Prussian War. Thomas limited himself to a few urgent measures, such as the elimination of the boarding component and the establishment of courses in aesthetics (required for composition students) and in music history, as well as a vocal ensemble (for the best voice students) and an orchestral ensemble; these “exercises publics” had begun during the early years of the Conservatoire, but had been suspended for more than ten years. Lastly, Thomas changed several regulations dating from 1850 that concerned advisory committees on instruction, the admissions auditions, and the examination committees for the classes.

Achille's first teachers were Antoine Marmontel for piano and Albert Lavignac for solfège; he enrolled in their classes on 25 October and 7 November 1872, respectively. Various accounts agree on his lack of punctuality, due no doubt to his idling in front of shop windows or lingering over the books and magazines in the bookshops. He often arrived at the last minute or even after class had already begun, “with short and hurried steps” or “completely out of breath from having run,” as Julien Tiersot recalled from 1876.