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 urgency of the situation by showing that the bus is approaching and Sylvia's desperation to get free, making the reader anxious about whether she will make it in time.