Yes precum can get her pregnant. But also the added stress can delay her period! I remember when I was in high school and exams were VERY stressful, often I would not get periods for weeks at a time! It scared me, but once I knew I wasn't pregnant, i was fine.
Go do a Home Pregnancy Test right away. If it is negative, then she is probably just really stressed. If you can't afford a preg test, there should be a "birth right" or some sort of sexual health clinic that could give her a free test.
Also, use a condom next time!! You're in high school, you DON'T want be worrying about pregnancy this summer, and a baby next summer...it's no fun!!
Your girlfriend could also get on Birth Control. Tell her not to be scared to talk to her parents (or mom) about it, i'm sure they'd accept her wanting to go on the pill rather than telling them she's pregnant!!
Good luck