It's the warlock's first time attending a magical school. In this school students are different magic users, but he is the only warlock. He has been repeatedly bullied throughout the school year; one time he is even injured. In the end he finishes the year and moves on to the next one.
He has studied spells and in the final demonstration casts a modified spell, which saves every one from dying due to the activation of a magical artefact (cylinder? orb?) by another student.
I would have read this story near 2015.