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 adds urgency and raises the stakes, as Sylvia is in a desperate situation while the bus she needs is arriving, which creates a heightened sense of tension.