How can one atone for stealing stories from classmates in childhood? And is it necessary to apologize to the teacher or buy new stories, or is seeking forgiveness sufficient?

A boy is not held accountable for his actions before reaching puberty, but he is liable for any property he takes from others or damages.

Therefore, you bear no sin for the books you stole from your classmates before puberty. However, you must return them if they are still available, or pay their value if they are damaged, or seek their forgiveness.

If you cannot reach the owners of the stories, donate their value as charity on their behalf. Strive to ascertain the value to be cautious and absolve yourself.

You are not required to inform the teacher or the owners of the stories about what happened. The obligation is to return the rights through indirect means, and a Muslim should conceal his own faults.