Mike Markel gives four situations in which the passive is to be preferred. In all four situations, the passive provides the desired sentence emphasis. Compare the sample sentences with versions converted to active voice. This rewrite focuses on the department, whereas the proper focus belongs on the students. John may have been at fault, but the only thing the sentence seems to be doing is assigning blame.
Passives also aid coherence among the sentences in a paragraph.
Look at the passives in the second paragraph of the Declaration of Independence. Notice how they lead the reader easily from one idea to the next. We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain inalienable Rights, that among these are Life, Liberty, and the pursuit of Happiness. That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed. Let's try changing the paragraph to active.