My thoughts
I think the film shows a determined, hardworking drummer being worn down by the pressure of an insatiable, cruel and bullying teacher. However good a drumming teacher he may be, his approach and behaviour take a heavy psychological toll. The injuries to the drummer's hands during practice make that visible. Another student also dies by suicide because of the teacher.
For me, the film is about that pressure and the state of a student who receives little support from his family. It also touches on the drummer's family relationships and love life; I read those as a troubled personal life and difficulty forming attachments.
I loved the jazz and the music throughout. But what kept me watching was the relationship between teacher and student, and the drummer's determination.