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 increases the sense of urgency and anxiety as it emphasizes Sylvia's struggle and the imminent arrival of the bus, which she is trying desperately to catch.