It's true that in Plato's dialogues he never speaks explicitly on behalf of himself. Still, there has been a wide consensus to interpret the dialogues such that the always-present character Socrates represents either the original Socrates (in the "early" dialogues) or Plato (in the "middle" and "late" dialogues).
In addition, the dialogues are not our only sources as to Plato's views. We also have the testimony of Aristotle, who spent about twenty years in Plato's presence, in the Academy. Aristotle discusses Plato's views in various works, usually criticizing these views, but also paying respect to Plato. Thus Aristotle discusses the Theory of Forms in the Metaphysics, the Unity of Goodness in the Ethics, and the Philosophical Republic in the Politics.
So you are correct in that in general we can and do attribute views from Plato's dialogues to Plato, and/or to Socrates. Still, I suppose that it is your professor's prerogative to require that in his course you put this aside, and just analyze the dialogues on their own, without adding the interpretation that they do, or do not, reflect Plato's own views.