Take a look at the original sentence again:
Bored with the lecture on comma splices and fused sentences, Jayson started
poking Kim with the sharp tip of his pencil finally
she whirled around and slugged him in the mouth.
You wanted to fix it this way:
Bored with the lecture on comma splices and fused sentences, Jayson started
poking Kim with the sharp tip of his pencil, finally
she whirled around and slugged him in the mouth.
To add a comma between pencil and finally would create an equally bad problem, a comma splice. A comma splice occurs when you have two main clauses joined with a comma alone. Jayson started poking Kim with the sharp tip of his pencil is the first main clause. Finally she whirled around and slugged him in the mouth is the second main clause. The spot between pencil and finally needs a stronger break than a wimpy comma.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.