Because tampons aren't meant to hold that much liquid. They're designed to absorb and retain, without staining the underwear or pants, small amounts of liquid. Also, she probably does use 'em already considering her age.
@Gladiator02
But who was it that made her the class president?
The class: 27 out of 32 students voted for her
Even the guy who knows of her condition voted for her ¯\_(ツ)_/¯