Robert Downey Jr. became good friends with James Spader, and even named his cat Jimmy in honor of him. They would appear together again in Unter Null (1987) and Avengers: Age of Ultron (2015).
James Spader is merely lip-synching during the piano sing; the song "We Walk the Night" is performed by Paul Carney, one of the sons of actor Art Carney.
The band in the movie is The Jim Carroll Band. Front man Jim Carroll wrote ''The Basketball Diaries'' (1978) that detailed his teenage drug years and was later made into the movie Jim Carroll - In den Straßen von New York (1995) with Leonardo DiCaprio. The band also has a song on the soundtrack of 'Tuff Turf' which was "People Who Died."
The original ending in the script had Morgan and Jimmy taking the SATs, and Morgan talking Frankie into taking them too, after she quips she isn't smart enough.