Characters can quit an extracirricular at any time. When a student quits an extracirricular, they are not longer able to use their Student Die for that club's skills.
When a student participates in an extracirricular, they gain a Student Die. A Student Die is a d4 the player can roll and add to an ability check that the character makes, provided the check uses one of the skills listed in the extracirricular's description. The player can wait until after rolling the d20 before rolling the Student Die, but must do so before learning if the check succeeds or fails. After rolling a Student Die, a player can't roll it again until the character finishes a long rest.
Students who engage in multiple extracirriculars do not gain extra Student Die, but they are able to choose from a broader list of skills that the Student Die can be used towards.