My son did this a couple of years ago -- he started with eyebrows (thank heavens he didn't do the eyelashes) and had very uneven eyebrows as a result. I definitely think it's a stress thing.
We also have had recent problems with his pulling his hair. His hairstyle is longer, like most teenage boys, and he pulls on it while testing, and it puts him into a sort of trance. I'm guessing this is stress-related as well.
I think the stress-ball idea is a good one. Something to squeeze or manipulate, with the added benefit that it is small and not easily visible as a "strange" thing by the other students.
I also think that, given time and adjustment to new routines, some of these things work themselves out on their own. Eventually.
Kris