The paragraph that would help build tension if added to the story is:
As she struggled to free herself, the bus pulled into view. She waved her arms wildly, but the driver didn’t notice her. And she was still stuck. What was she going to do? She could not be late to school today.
This paragraph heightens the tension by showing the urgency of Sylvia's situation, emphasizing her struggle and the impending arrival of the bus, which adds to the pressure of her possibly being late.