Passive voice isn't generally a no-no, at least at my school. Passive voice is usually discourage by teachers because sometimes it creates confusion: 'the cat was killed' doesn't tell me who killed the cat but 'Tony Abbott killed the cat' makes it clear. Sometimes it also doesn't 'sound as good' but that's pretty sure he give and depends heavily on where you're using it. The best thing to do is, if possible, ask your teacher whether they mind it, especially if they'll be marking your exams and essays.