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Hi everybody

Please correct the following short story for me. 
Thanks.

One day, Nicole, a little girl, was walking home and eating an ice-cream cone at the same time. She did not notice a banana skin on the ground. She stepped on it and fell down heavily. Her knee was wounded. 

She cried, “Help! Help! My knee is bleeding. Someone help me, please.” But nobody was around!.

About three minutes later, she could hardly get up. She was very sad. A short while later, with much difficulty, she forced herself to get up and trudged home. When she opened the door, her mother was upset when she saw that she was injured. She quickly washed the wound and took her to the clinic in a taxi. The doctor cleansed the wound, gave her an injection, prescribed some medicine, and gave her two days of medical leave. 

After her mother had paid the bill to the clinic assistant, collected the medicine and the medical certificate, they took a taxi home.
Comments  
It seems fine to me, but I'd change some words.

One day, Nicole, a little girl, was walking home and eating an ice-cream cone at the same time. She did not notice a banana "PEEL" on the ground. She stepped on it and fell down SO HARD. Her knee was wounded.

She cried OUT and SHOUTED: “Help! Help! My knee is bleeding. ANYBODY there! help me, please.” But nobody was around!.

About three minutes later, she could hardly get up. She was very sad. A short while later, with much difficulty, she forced herself to get up and trudged home. When she opened the door, her mother was upset TO SEE HER DAUGHTER'S KNEE WOUNDED. She quickly washed the wound and took her to the clinic BY TAXI. The doctor cleaned the wound, gave her an injection, prescribed some medicine, and gave her two days OFF.

After her mother had paid the bill to the clinic assistant, they collected the medicine and the medical certificate, they took a taxi home.

I'm not a native speaker, but tried to help as much as I could. As for me is ok!

Regards from Chile!
Thanks, Chile.Emotion: smile

As long as no native speaker disagrees with your corrections, I think you are on the right track.
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This is my revised version to improve the fluency:

One day, Nicole, a little girl, was walking home [while] eating an ice-cream cone at the same time. She did not notice a banana [peel] on the ground and stepped on it. As a result, she fell terribly and injured her knee.

She cried out loudly. “Help! Help! My knee is bleeding. Someone help me, please.” But nobody was around!.

Minutes passed. She still couldn’t get up. She was helplessly sad. After she rested for a short while, she gathered enough strength to get herself up but not without difficulty. Finally she was able to trudge home. When she opened the door, her mother was upset when she saw Nicole’s injury. She quickly washed the wound and took Nicole to the clinic in a taxi. The doctor examined her wound and gave her an injection. After dressing her wound, he gave her a prescription for some medicine and a two-day doctor's note.
After Nicole’s mother paid for the medical treatment and picked up the medicine from the pharmacy, they went home on a taxi.
Thanks, Goodman

After Nicole’s mother paid for the medical treatment and picked up the medicine from the pharmacy, they went home on a taxi..
Regarding the above sentence, I would like to confirm about 'on a taxi'. Is it the same as on the bus? I often hear people say in a taxi.
I appreciate your help. Emotion: smile

Thanks, Goodman.

Regarding the last sentence 'After Nicole’s mother paid for the medical treatment and picked up the medicine from the pharmacy, they went home on a taxi.' Is it the same as 'on the bus'? I often hear people say 'in a taxi'. So they are wrong.

I appreciate your help. Emotion: smile
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Thanks Emotion: smile