Here Comes the Boom (2012) on Netflix

Plot summary

Scott Voss is 42 and has lost his enthusiasm for teaching biology at Wilkinson High. When budget cutbacks put the music program in danger, and with it the job of its teacher, Marty Streb, Scott looks for a way to raise the $48,000 the program requires. He takes a second job teaching an adult citizenship class, where one of his students, Niko, asks for extra help outside class. At Niko's apartment Scott discovers that Niko once fought in mixed martial arts, and he learns that a fighter is paid $10,000 even for a loss. Scott decides to step into the ring himself. Niko and Marty help him train, and he takes small unsanctioned bouts, while he also tries to get the attention of the school nurse, Bella Flores.