Posters which they displayed in the public library were designed by the students.
In the first sentence, "Posters which they displayed in the public library" is taken as its subject and the voice change is done accordingly.
Take a look at the second sentence-
A beach cleanup was undertaken and over 15 sacks of garbage were being collected by members of the school's Environmental Club.
I have changed both "bench cleanup" and "garbage" sections into passive form to make it meaningful.