I'm just also gonna put my reply from on my profile here too to keep everything in one place :)
To add onto that, I think you and your friend should have a conversation and you should tell her that the responsible thing to do is to allow you to find a new home for this duckling. Maybe...
If you really can't get another duck, and you go to school and she's alone 7 hours a day, please consider rehoming her to a place that can have more ducklings with her. Keeping her alone is just not fair to her. She is a flock animal and she needs the company of other ducks.
It's the best for...