Your co-worker seems to get special treatment all the time, and it drives you crazy. Now, it's possible that your boss is simply a bad manager who plays favorites (or who is too wimpy to stand up to an employee who doesn't take her job seriously). However, while some managers totally stink, most are doing the best they can and have very good reasons for giving your co-worker privileges that you don't have. Here's why her life seems to be better than yours:
She negotiated it when she was hired.
Most people know they can negotiate salary, but you can pretty much negotiate anything. Some things are easier to get than others--for instance, getting an extra week of vacation when you're not an executive is practically impossible in many companies--but go ahead and ask. Things like flexible schedules, however, are becoming more and more common.
If you want to come in early and leave early so you can be home when the kids get back from school, you can try to negotiate this. If you're a night owl who works best after 10:00 a.m., you can negotiate this as well. Not all bosses will say yes to everything, of course, but some will.
Her salary is cut accordingly.
Some people value flexibility over money. Some people want a 35-hour workweek instead of a 40-hour workweek and are willing to be paid for 35 hours instead of 40 to make that work. Your co-worker may be working fewer hours than you are, but she may be getting paid a heck of a lot less than you are for the privilege.
She's a superstar performer.
Yes, top performers get privileges the rest of us don't. If your boss is smart, she'll let your super productive co-worker come and go as she pleases. We learned in school that being on time to class and doing the busy work is just as important as getting an A on the test, but in the real world, this just isn't true. Smart bosses let their employees work independently as long as they are performing at a high level.
She has a disability covered by ADA.
Your co-worker may leave early frequently because she has chronic migraines. She may take every Friday off because she's covered by intermittent FMLA and has chemotherapy every Friday. She may be perfectly healthy, and still have every Friday off--sanctioned by FMLA--because she's taking her mother in for chemotherapy.
Not every flexible schedule or privilege is for fun things. The Americans With Disabilities Act and the Family Medical Leave Act both require businesses to offer certain accommodations to people who have serious illnesses or are taking care of people with serious illnesses. She may not be talking about it because it's none of your business.
She's working when you're not watching.
You may think your co-worker is a huge slacker because she's never in the office, but she may be working from home. She may leave at 3:00 to be home when the kids get back from school, but then she works from 8 p.m. to midnight every evening, meaning that she's putting in more hours than you are.
She may have different responsibilities that require her to deal with a global clientele, and it makes no sense for her to come in at 8 a.m. when it's 4 a.m. where her clients are.
How can you get these perks?
This may seem blindingly easy--but ask. Go to your boss and say, "I'd really like to introduce some flexibility into my schedule. Is it possible for me to work at home on Tuesdays?" See what she says. If you're a top performer that she can trust, you'll probably get a yes. If you're a mediocre performer, you may be out of luck. In that case, ask what it is that you need to do to receive increased flexibility. And then shut up and listen to your boss.