Maybe she stepped on something sharp like a stone or a thorn. It could be really painful and that's why she regretted it.
She might have walked in a dirty place and got her feet really muddy or contaminated. For example, if she walked through a muddy puddle and then had to clean her feet thoroughly, which was a hassle, she would regret going barefoot.
Maybe she lost her shoes. It could be that they got stolen or she accidentally left them somewhere.
She might start to value shoes more. After going barefoot and facing problems, she realizes that shoes can protect her feet from harm.
The teacher might be shocked at first. Just seeing a student without shoes in class is not a common sight.
One reason could be that she was playing a game with her siblings or friends. They might have decided as part of the game rules to immobilize her and tickle her for fun. For example, a game of 'capture the flag' where if she was caught, this was her 'punishment'.
There was this girl. One day, she woke up early and went out barefoot into the garden. The dew on the grass wet her feet but she didn't mind. She started to pick the fresh flowers for her mother. As she walked around the garden, she could feel the earth beneath her feet, grounding her and making her feel at peace. This was her special barefoot story.
It's possible that she had some kind of accident with her shoes. For instance, the heel broke or the strap snapped, and instead of going home to change, she decided to continue shopping barefoot.
She could be barefoot because she was playing a game like hide - and - seek outdoors and got stuck in a bush while trying to hide.
One reason could be that she stepped on something sticky like gum on the sidewalk. It held her shoe firmly and rather than struggle with a dirty shoe, she took it off and was stuck barefoot.
It could be that someone played a prank on her and hid her shoes. Some students like to do naughty things like that. There's also a possibility that her shoes were old and the sole got detached suddenly, making it impossible for her to wear them.
Well, she could get stuck because she might be in an unkempt area like a field full of holes or a muddy patch. And as for being tickled, it could be that a friend or a sibling is around and they see her stuck situation as a chance to be playful and tickle her.