Do you recall the scene in Mr Bixler's Adult Poetry class? There's a scene where Rhonda and Justin take everyone's hands and begin their spiritual work. Just before they begin to hold hands, a woman says, "That's really cool!". That woman is Jackie.
Find that scene, pause that scene and look at a pic of Nancy Chlumsky. It's her.
This really should be on the movie's Trivia page on IMDB.
From the movie script linked above (She's listed as "WOMAN" as I've pointed out below):
SUMMER WRITING CLASS DAY
Before the class started, Ronda and Justin
wanted to lead the class in a group
Jackie --> WOMAN
Ooooh that's really cool.
Okay what we're gonna do is, send our vibes
out into the group.
Justin turns on some kind of tape deck which begins to play
Everybody hold hands, and close your eyes.
Relax your muscles and take deep breaths.
Now, try to feel what the other person is
feeling, without speaking any words, send out
your vibe, and receive the vibes around you at
the same time. Can you feel it?