The bag is taken outside for the warm-up! The magical size bracelet chapter 6 by SatireMarker

You gasped in imagined panic as you felt the bag being picked up and taken out onto the field. You struggled to maintain your footing as you tumbled about inside it. You rolled into the side of the heavy bag, and a stack of training cones slide towards you. You decided to play the role properly and at least try to get out of the way and rolled to the side, barely avoiding their would-be crushing impact; you wondered if the sliding cones would have killed you were it not for your invincibility.

Before long, the bag was placed down and opened up. You buried yourself in the corner and tried to ensure you weren't noticed.