Within the English classroom, active voice is most frequently taught as the accepted and proper form of expression. In active voice the subject of the sentence performs the action. Passive voice as Pohnpei has indicated does have a purpose, but I suspect, that in the high school classroom at least, the rhetorical purpose for using passive voice is rarely taught.
The main reason for using the passive voice is a desire to put less emphasis on the action and on who did the action. This is why you will hear politicians say "Mistakes were made" instead of "I made mistakes." By using the passive voice, the speaker or writer hopes to deemphasize the action and especially to deemphasize who committed the act.