Dear I-doubt-you're-a-simpleton, The passive voice is used (and even regarded as mandatory) in scientific or technical writing or lab reports, where the actor is not really important but the process or principle being described is of ultimate importance. Do not mix and match. Be consistent and, if you see your HS teacher again, tell him or her that their advice was correct for everything BUT computer, medical or other technical writing.
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Apparently, even in science/medical writing they are moving away from passive now. At least according to this artcicle.