Take another look at the sentence:
Zipping his book bag closed, Pedro's arms folded and his eyes closed, even
though his math professor continued solving problems at the board.
Zipping his book bag closed, a participle phrase, comes right before arms, an illogical target. [Pedro's, a possessive noun, functions as an adjective describing arms.] The whole Pedro, not his detached arms, zipped up that book bag! Before you continue, review the rules for misplaced and dangling modifiers.
Go back to the sentence to try again.