The Substitute 2: School's Out (1998) on Netflix

Plot summary

Randall Thomasson is shot while trying to stop a carjacking in Brooklyn. His brother Karl, a mercenary, comes to the funeral and meets his teenage niece Anya, who is angry that he stayed away from her and her father. Karl sets out to learn who killed Randall, and Lenthrop High School, where Randall taught history, gives him his way in: he signs on as a substitute teacher there. The faculty is cynical, the students are violent and disruptive, and the carjackings are blamed on a street gang called the Brotherhood, led by Little B. A teacher, Kara Lavelle, helps him, and he teams up with his fellow mercenary Joey 6 and befriends a school janitor who is also a war veteran. Karl also protects Anya.