It's a movie about a teenage boy and girl who rob a bank on their way to school. The police show up so they can't leave the bank and the teenage girl age girl catches her period for the first time.

Pups (1999)
Starring Mischa Barton, Cameron Van Hoy, Burt Reynolds
IMDb synopsis:
Two teens impulsively decide to rob a bank, a la Bonnie and Clyde. Quickly they find themselves in over their heads as they are forced to take a bank full of hostages and the FBI are at the door. Hostages include a heroin addicted Gulf War vet, the pesky bank manager, a gruff old man, a calm bank employee, and a helpful young woman. The head FBI man must sort through the psychology of all the participants to try to keep the events from escalating. Underlying in all of this is the pure adolescence of the two who initiate the trouble.

Pups (1999) - IMDb (http://www.imdb.com/title/tt0185101/)