How did Tracy know about Mr. McAllister and his wife's inability to conceive? Dave told her. According to interpretation, she either fished it out of him or he was so besotted with her that he overshared on private matters.
Mr. McAllister believes that Tracy sleeps with older men because she wants to use them to climb to the top, but, assuming that she is sleeping with the congressman McAllister glimpsed while on his trip to DC and that she willingly entered a relationship with Dave Novotny, it's possible that Tracy prefers older men for romantic relationships because she is unable to relate to people her own age, and/or that this is the aftermath of being groomed at a young age.
The other side of logic here is that Mr. McAllister is so filled with hatred towards Tracy that he is attempting to put any negative characteristic on her that he can.
There's also the fact that if you're a young female intern and your boss the congressman starts putting the moves on you, sleeping with him may or may not advance your career, but telling him where to stick it will almost definitely stop it dead in its tracks. Given that McAllister spent the entire movie trying to ruin Tracy's life when he wasn't even the one who was sleeping with her, though, he'd probably have no problem with that.
Because the movie is in Mr. McAllister's point of view, Tracy is portrayed as an annoying Persona Non Grata among the students, but the fact that half of the students at the school voted for her shows that some of the other students seem to have some good opinion of her, or that at least they believed she would do a better job as class president than Paul. Tracy's friendlessness seems to be more a side effect of her busy schedule and focused ambition rather than because the other students actively shun her.
