Review the sentence:
Zipping his book bag closed, Pedro's arms folded and his eyes closed
while Professor Johnson continued solving algebra 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.