First off, it's awesome that you're going to try to breastfeed after hearing so many horror stories. I heard a bunch of horror stories too, but after almost 3 months my son and I have a wonderful nursing relationship and I wouldn't trade it for the world. When he was 3 days old he had to go to the hospital and they did a lot of blood draws, IVs, and so on. The one thing that kept me sane was being able to nurse him when he cried. Today he had his vaccines, and was crying a lot, but he calms down when he nurses. I love that I am able to do that for him. I also love the cute 'milk smiles' I get when he'll be nursing and all of a sudden look up at me and smile, with milk dripping down his face, then get back to work nursing.
All of that said, things didn't start easy. My nipples hurt because he wasn't latched on properly. I got help from a breastfeeding support group, and my only regret was that I didn't try to get help earlier. So, you hear horror stories, but a lot of what hurts is when it's not done right, and most of that can be fixed with the help of more experienced women. My best mommy friend (you gotta get yourself some mommy friends now, mama!) never had any pain or discomfort -- she grew up on a farm and said that she put her boob in her babies mouth the same way she put a bit in a horses mouth... get it in FAST!
I agree with jruck's comment that it may be possible to pump at school -- talk to a school councelor or pricipal to see if they can arrange for you to pump, at the very least at lunch but if possible twice or three times per day with how long you'll be away from your child. If you get "no" the first time, go talk to someone else -- the school nurse, your own doctor or midwife (you might be able to get a doctors note saying that you need to pump), whoever you can think of who might advocate for you and your daughter. I had to fight a bit of a war to get time to pump at work, and I can't imagine having to do that at 15, but you're a mom now whatever your age, and it will give you a power you never knew before!
If your family is rich, you might want to see if you can't convince your mom or dad to make a few appointments with a lactation consultant ahead of time (maybe 3 days after the baby is born, 10 days after the baby is born, and 3 weeks after the baby is born). If you don't have any problems, you can chat it up with a supportive person for a half hour or an hour. If you do have problems, you can get them addressed fast. Plus, once you have a lactation consultant you are seeing, most of them will give you support on the phone whenever you need it. If your parents won't go for that, try the La Leche League or similar group. I mention the lactation consultant option only because most LLL meetings are during the day; you might find it easier to get excused from school to see a health professional.
Avoid formula if you can... but if you need it, use it, and keep in mind that any breastmilk you can give your baby is worthwhile, be it one week or three years, be it liters or teastpoonfuls.
Best of luck mama! Have a great pregnancy, a wonderful birth, and peaceful new-mommy-hood.