Oh this happens to me all the time, two hours ago a bunch of my friends told me to shut up because I was discussing the metric system with someone.
Honestly I wouldn't take stuff like this seriously, the people who avoid using their minds in life often end up with very poor ones. And it definitely shouldn't be something you try and cover up for, if people aren't willing to realize that intelligence is a virtue, well, fuck'em.
As Bill Gates used to say: "Be nice to nerds, you'll probably end up working for them."
Of course, I don't really know the full story. Maybe she studied her ass off for that exam and then got a lousy mark, or she was completely lazy. Either way, it's just easy to chalk it up to hormones (ah, I love blaming every stupid thing teenagers do on hormones) and shrug it off.