Thanks for the advice everyone. People, not just me, have made little comments to the boss and he ignores it. If she had some sort of alternate schedule for school, or personal reasons, he's cool enough that he would say "She's allowed to leave early every day, she has it worked out with me." If he said that, no one would ever mention it again. About the firing thing, our boss fired one of my co-workers who had been working there for like 2 years because he thought he had taken a day off, but it wasn't on my bosses calendar, so when he didn't show up that day, my boss fired him.
It's gotten to the point that no one else will speak to this girl. We all hate her because she gets away with everything. We would all love to start leaving early too, but we're too responsible for that. My co-workers and I have decided to take it into our own hands...let's just say her timecard now reflects the true hours she works. She can cheat the system...well so can we