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 urgency of the situation and emphasizes Sylvia's desperation as she tries to escape from the fence while the bus, which she desperately needs, is about to leave without her.