In a story included in one of the Chicken Soup for the Soul series, the author talks about his experience of talking to his grandmother about a revenge he was going to take on the eighth-grade bully, who had punched him in the stomach. She talked him out of his plan, saying, "Good deeds beget good results, and evil deeds beget bad results." Then he gives an episode in which he realized how his grandmother's good deed was repaid. And he concludes his story with:
What about the eighth-grade bully?
He ran into the ninth-grade bully.

(You can read the full story at: http://www.chickensoup.com/books/soup_2_sample_01.html )

Does the last sentence mean/imply that the eighth-grade bully ran into the ninth-grade bully when he was still in the eighth grade? I.e., did he encounter a bully one year older than him? Or does it mean/imply the kid who was the eighth-grade bully encountered a bully when he was in the ninth grade? I.e., did the first kid run into a bully of the same age the next year?

Also, I noticed that the word "bully" is modified by "the", not by "a", from the beginning. Why is that? Is it generally understood that there is only one bully in each grade?

Thank you in advance for your help.

It implies that he got his come-uppance at the hands of a bigger bully, yes.

Not sure about the "the". Maybe it's a human/biology thing - like, there can only be one king, one leader of the pack, one alpha male. Just a thought. [:^)]

Thank you again, Rommie!