When I was 11 I attended an international school in Madrid, where I happened to be the only Turk. I was an introverted, ill-fitting child, unable to keep up with the effervescence of my peers.

During lunchtimes I sat alone in a corner and read stories. It didn’t take the other kids long to start making fun of my awkward ways. Before I knew, I was the darling of the bullies. It didn’t help that recently a Turkish terrorist had tried to kill the pope.

When I entered the classroom, the boys made the cross as though I were a vampire. Nor did it help that Turkey got zero points at Eurovision that year. The boys nicknamed me Opera, after the song that had received no points. I am well aware that had I been a cool kid, one who knew how to dress up, crack jokes or dance, I could have overcome the stereotypes. I couldn’t.

At that school there was a subtle hierarchy of nationalities. The most popular kids were usually Dutch, Swedish, British or German. Towards the bottom of the list were the Others: “the untouchables”. Turk, Bangladeshi, Pakistani, Persian. There was also an Indian girl, thin as a rake, quiet and shy. Her name was Kiran.

One afternoon in the lunch room I heard a ruckus. We had returned from volleyball, and I had placed my sports bag at one end of the table. The month before I had started having my period. Now I saw in horror that one of the bullies had opened my bag and taken out a sanitary pad, which he was waving in his hand like a bandanna. I was mortified.

“Whose bag is this?” he shouted.

A few people stared at me.

“Are these pads yours?” the bully asked cheerfully.

Silence. Seconds that felt like hours. “No,” I said slowly. “They belong to Kiran. She had asked me to keep them for her.”

The same afternoon, back in the classroom, when the boys were mocking Kiran, I can never forget the look in her big, brown, beautiful eyes. She could have been my friend, my sister, and yet I had hurt her. Just because she was one of the “unpopular kids” – like me.

I wanted to tell her how sorry I was. Every day I promised myself I must talk to her, but my shame was too deep. A few months later she left the school; her father had been appointed elsewhere. Suddenly, she was gone. She left behind a huge burden on my conscience, one that I still carry with me.

Elif Shafak is an award-winning author of books including The Bastard of Istanbul, Forty Rules of Love and Black Milk: On Writing, Motherhood and the Harem Within