Conversation so far:
Who is Busoni in relation to Kurt Weill?
After examining some of Kurt Weill's compositions, Busoni accepted him as one of five master students in composition at the Preußische Akademie der Künste in Berlin.

Question: When was that?
Self-contained rewrite: When was that when Busoni accepted Kurt Weill as one of five master students in composition at the Preußische Akademie der Künste in Berlin?