In Oleanna, an instance of sexual harassment is depicted which is all the more typical precisely because some observers would see an ambiguity about it or perhaps would not recognize that it involves issues relating to the traditional, patriarchal dynamic that has tended to exist between men and women.
When Carol comes to John after class to question her grade on an assignment, she is basically asking him for help because she's had trouble understanding the course material. But instead of being sympathetic, John begins talking down to her, speaking rapidly in a deliberately authoritarian manner, as if more interested in displaying his own brilliance than in helping her. He reads back to her a sentence from the paper she's written and then, with a look of exasperation, says, "Now what can that mean?" He may not be intentionally humiliating her (though he probably is), but this is the result, given the impersonal and condescending way he's speaking.
Carol is clearly upset and flustered. One would think John's role as a teacher would be, quite simply, to teach—in this situation after class just as he does in class. By going on and on in his machine-gun-like delivery, he's intimidating rather than being instructive and helpful as he should be. The impression is that he simply wants to get her out of the way so that he can return to his focus on the issues involved in the house he and his wife are buying. His impatience takes the form of a kind of bullying. At one point when Carol gets especially agitated, John grabs her, saying "Easy!" as if amused by the plight of a nervous female.
Up to this point of physical contact, a skeptic might ask if there was really anything inherently sexual in John's treatment of her. One could hypothetically imagine the student asking for help as a young man rather than a young woman and ask if John would have behaved the same way. It's possible he would have, but unlikely. In a male-to-male confrontation, a professor might be abrupt and negative, but not in the amused, lording-it-over manner in which he treats Carol. He's clearly taking advantage of an assumed power dynamic that is supposed to exist between men and women, and this is the essence of sexual harassment, regardless of whether any overtly sexual gestures have been made.