1 The infinitive isn't marked to person in English (or in any other language I know of - except Portuguese [of which I'm an ardent fan]. So you 'should have said': 'The students will be given a piece of paper so that they can make a poster.'
2 (More likely) The plan should - in the view of your teacher - have specified subjects for active verbs: 'I will give the students a piece of paper so that they can make a poster.'
I called these 'possible objections'; the sentence as it stands seems quite acceptable to me though. If the teacher prefers you to use the active voice in your plans, she should have said so.