1. Our classroom has been decorated by my classmates.
2. All the biscuits have been eaten by children.
3. This land has been bought by property developers.
4. A hand hasn’t been asked to be given to them.
5. A lot of good work has been done by charities in this country.
6. The new bridge has not been build by them yet.
I tried my best, but there are different ways of wording these sentences in passive voice, I hope it helped anyway :)