She played a high school friend of Buddy's, who committed suicide. I remember being really shocked because teen suicide just wasn't depicted on a TV shown then. Buddy took it very much to heart and felt it was all her fault for not being a good-enough friend. This was in the later episodes and not in the DVD set released last year.
Yes, that was from "The Competition". Kristy's performance was outstanding...her look of shock and disbelief when she pays her condolences to the girl's mom, who only congratulates her on her history competition win! Her depression over the incident and stubborness on refusing to see that the girl's suicide wasn't her fault.
Doug handled it though:
Doug: That poor girl had probably been living in a pressure cooker most of her life. This history contest was just one in an endless series of contests she felt she had to win, every day. People don't usually kill themselves for one failure, but because of a prevailing sense of failure on every front. Particularly, a failure to win love.
Buddy: Whose love?
Doug: Her parents'... Her mother's. Which is one thing you will never have to contend for.
I guess it was Disc 2, the one where Buddy and her entire class have to give a speech on where they will leave their mark on the world. something like that. In any event, both Dana Plato and Helen Hunt played her classmates.