A small amount of semen is released before the guy actually ejaculates, which could also get you pregnant, so yes, it is entirely possible. Only one sperm has to be a good enough swimmer to get you pregnant.
When adult couples are trying for a baby, it often takes months of trying before the woman conceives. So when you look at it like that, just because you could be pregnant, it doesn't mean you are.
When did this happen? If it was within the past 72 hours, I suggest you get the morning after pill as soon as possible. If it was within the past 4-5 days, you could get an IUD fitted, which could also prevent a possible pregnancy. Obviously you'd have to talk to your doctor about this, and I don't think it's standard procedure to fit them for young girls, as your body may not have finished growing.
Other than that, the only thing you can do is wait for about three weeks, and then take a pregnancy test. I don't think going to your doctor right now (unless for an IUD) will help, as it will be too soon for them to tell whether you are pregnant or not. You might want to visit planned parenthood or your local Brooks centre, as they will be able to give you all the advice you need.
I know it's difficult, but please try not to panic. The only thing you can do (assuming this happened within the last couple of days) is to get some emergency contraception as soon as possible. Other than that, you'll have to wait. If your period comes in two weeks' time, great. If it doesn't, don't panic straight away. Stress can often delay your periods, and if you're worried about possibly being pregnant, it probably won't come.
If you find out you are pregnant, the first thing you need to do is see your doctor, or go to your local family planning clinic. They will be able to go through all your options with you. It's best to go as soon as possible, because the sooner you acknowledge the fact that you are pregnant, the more time you have to decide what you are going to do. If you decide to have an abortion, it is better to have it done as soon as possible. If you decide to have the baby, you'll have longer to come to terms with it.
If you're not pregnant, you can breathe a huge sigh of relief and learn from this. There is really no excuse for accidental pregnancy, as contraception is free on the NHS and extremely effective if you use it properly. Hormonal birth control, assuming there is no reason why you can't use it, is definitely the safest way of preventing pregnancy, and you should always use a condom to make doubly sure, as well as to prevent STDs. You might think you know the guy, but plenty of girls have been fooled by someone they thought was nice. Even if you get yourselves tested together, a guy could cheat on his girlfriend, pick up an STD and pass it on to her. So it is really safer to use a condom at all times until you're in an extremely committed relationship. The fact that you had unprotected sex to begin with suggests to me that you didn't take the precaution of getting yourselves tested, and you will need to do this as well. The family planning clinic is a good first step.
I hope you're not pregnant. I really do. But if you're going to continue to be sexually active, please be more responsible next time! Get yourself tested for STDs, make sure you're clean, get yourself on the pill (or another form of hormonal contraception) and always use a condom. You'll enjoy sex much more if you're not constantly facing pregnancy scares!