I don’t think the teacher is necessarily wrong, actually. For example, in my calculus class, I’ve always made it clear that everything we do is being done in the real numbers, there are no complex numbers used in the class. Any function’s domain and range are subsets of the real numbers, and that’s the “setting” where we work. In that case, I would count off if the graph of the above function was given as the whole parabola y=x[sup]2[/sup], because the negative reals are not in the domain. If, however, your teacher was never clear on your “setting”, then you may have an argument.