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 addition heightens the sense of urgency and panic Sylvia is experiencing, as she realizes the bus is about to leave without her while she is desperately trying to escape the situation with her backpack.