She dove for the ball but landed on her knee funny.
What did you do when you watched her fall?
I started laughing at her.
Seeing this girl fall on her knee is no call for you to laugh at her. Help her up, dust her off and support her instead.
I wanted to teach her a lesson.
I don't think laughing at someone's misfortunes is teaching them a lesson. How would you feel if you were her in that situation?