That is not what we mean by nonconsensual. It's a similar theory to when children are molested - sure, they can say yes, but they aren't in a position to say no because they have no power in the situation.
Even if she is his same age, and pursued him, the power balance is not equal and therefore the relationship cannot be considered consensual. She has everything to lose - scholarship, playing time, etc. He holds power over her, similar to a boss at a company with a subordinate. It's unethical to sleep with a subordinate, no matter how much they flirt with you. The power balance is off. That's why universities and workplaces have rules against this.