The simplest solution to the bridge challenge is to have the agent go back and forth across the bridge.
This solution essentially has three components:
The code block to give the agent the blocks to build one line across the bridge,
The code block to turn right and return across the bridge
The code block to turn left and reset the position to cross the bridge again.
The "set block or item" could go at the beginning outside of the repeat blocks, but the agent will require more than one stack of dark oak planks to complete the bridge.
Talking to the NPC will give the students the button option to reset the challenge and clear the space between the bridge ropes.
Students can document their final working solution by resetting the challenge and using screen capture to document their code and its result.
Note: After recording this, I realized that since it's inside the 21 repeat loop, I only needed to give the agent 1 block each time.