To try and convince director Destin Daniel Cretton to cast her Brie Larson told him that she had applied to volunteer with disadvantaged children after reading the script in order to research the role. Cretton was impressed and Larson did not reveal until later that she had been rejected by every organization she had applied to.
While most of the children were cast through open casting calls, Keith Stanfield was the only actor to return from the original short film Short Term 12 (2008). Director Destin Daniel Cretton had to struggle to reach him during the casting as he had quit acting and did not have a cell phone.
The song featured in the film, "So You Know What It's Like," was originally written by writer/director Destin Daniel Cretton and then re-written by Keith Stanfield, who is also a rapper and sings the song in the film, in an attempt to incorporate his own experience as a child into the song and make it more effective.