Why was Gilbert in Anne's class although several years older? What did he do to earn Anne's scorn 'forever'?

Gilbert was behind is school because he went to Alberta with his father while he was ill. He rarely attended school while there, thus, he fell behind.

"You'll have Gilbert in your class after this," said Diana, "and he's used to being head of his class, I can tell you. He's only in the fourth book although he's nearly fourteen. Four years ago his father was sick and had to go out to Alberta for his health and Gilbert went with him. They were there three years and Gil didn't go to school hardly any until they came back. You won't find it so easy to keep head after this, Anne."

Gilbert teases Anne during class, and she responds by hitting over the head with her slate. Anne is punished even after Gilbert confesses her part.... she is forced to stand on a platform for the entire afternoon. She vows never to forgive him.