[Your ability to write English is excellent. Therefore,  much of my suggested changes are stylistic for an English speaking audience. Review the rules regarding independent clauses and complete sentences. If you have two complete clauses (both having a subject and a verb) they need to be separated in a proper way. And since nothing is ever easy in English :0) , this could be done by making two separate sentences or by using a conjunction (, and) or by the use of a semi-colon. And I sure hope that was your worst year. Sounds painful!]]

When I was an eleven-year-old boy, my nose was broken by a friend of mine. I was playing, when all of a sudden a long, narrow piece of wood hit my face just under my left eye. I learned later that my friend had thrown it toward me, but he had not intended to hit me. I could see out of my eye, but my nose was broken. My face started to bruise and swell.

My father took me to a clinic and a female doctor put a metal bar in the left hole of my nose to fix the broken bone. Of course she used anesthetic. She casted it afterward.

After one week I received permission to go back to school. At the end of my first day back at school, my father came to pick me up. I told him I wanted to walk home with my friends. He reluctantly let me do this, but only after I persisted.

Then, one of the saddest events of my childhood happened. Just before arriving safely at my house, my classmate twisted his backpack above his head and, unfortunately, struck my nose with it. So I was sent to clinic for the second time.

An important event in my childhood.

I can remember when I was eleven years old, a friend of mine broke my nose. I was only playing and then I felt a narrow, long piece of timber hit me just under my left eye. Somebody had thrown it towards me. This meant that my eye was fine and I could still could see but my nose was broken. As a result of that, my face became bruise and swollen. 

My father took me to a clinic and a female doctor put a metal bar in the left hole of my nose to fix the broken bone. Of course she used anesthetic. Finally she put a cast on it.

After one week, I was granted permission to return to school. Even though my father came to pick me up, I protested I wanted to him walk home with my friends. Although he wasn't please about it, I insisted.

Then the important event of my childhood happened. Near our house, my classmate was swinging his backpack above his head and unfortunately it hit my nose again, so I was sent back to the clinic for the second time.