Here's the quote from the original source[0], it doesn't make it entirely clear but may add some context:

> Later at Stanford, Mayer found herself in a group setting that was less social, more comfortable, and more familiar for her. As an upperclassman in symbolic systems, she was tapped to teach a class.

> She took to it naturally.

> Computer science professor Eric Roberts, still Mayer’s mentor, supervised her teaching. He says she was “unusually good at it” and “extremely effective.”

> After Mayer taught a course in the spring, Roberts took a survey of her students. The results were astounding: They loved her — even if she did sometimes talk “a mile a minute.”

> Roberts asked Mayer to stick around Stanford to teach another class over the summer; she readily agreed.

[0] https://web.archive.org/web/20130826113249/http://www.busine...

According to Wikipedia, it seems accurate.

> During her junior year, she taught a class in symbolic systems, with Eric S. Roberts as her supervisor. The class was so well received by students that Roberts asked Mayer to teach another class over the summer.

as a Stanford alum, I can tell you that undergrads don't teach classes at Stanford. At most she was a TA or was allowed to give a lecture. Outstanding grads are sometimes given the opportunity to teach whole undergrad courses, but only during summer. More rarely, students may give lectures (uncredited) while their professor is present in the class room. I'd love to see a link to a Stanford course page if that's not the case.

You're right, she was only a "teaching assistant for some course"

http://news.stanford.edu/news/1999/june16/taawards-616.html