If you have sex with someone during high school lunch period, you will not hear the end of it for the rest of your life. Try this: think of the most embarassing thing you did when you were twelve, and how much you wish you hadn't done it now, and multiply that by ten. That's how you'll feel about this in a couple of years.
I know that everyone "knows" that their first boyfriend/girlfriend/whatever is "the one" and that they'll be together for the rest of their lives and all, but it's just not true. I don't know of a single relationship which has started at the age of sixteen and endured; even the ones who get married right out of highschool usually end up divorced in a couple of years. Chances are that this guy is going to break up with you, no matter how strongly you (and he) believe that it won't happen, and afterwards your life is going to be a living hell. You will either be afraid of entering relationships with anyone for a while and later develop a cynical outlook on love from your experience, or perhaps worse you will begin using your body as a bargaining tool to try to get the guys who you want.
Now if you want this just for the physical experience -- if you don't care if your boyfriend breaks up with you the next day and tells all his and your friends what happened, then go ahead and do it. I know girls who really do feel that way, and that's the decision that they make and they're happy about it. However, if you want your relationships based on love rather than just an easy way to get sex when you want it, this is not something that you should do.
Of course, there are issues other than these if you're a religious person, so if that's an issue for you you might want to talk to a friend you trust who goes to church/synagogue/whatever with you. (Obviously you probably don't want to discuss this with your parents.)
No matter what happens, you're going to be experiencing a very emotional moment of one type or another next Monday. Remember that no matter what happens there are people that love you and care about you, and remember that no matter what you do it doesn't set a precedent for the future. You don't have to have sex because you had sex before, and you don't have to say no just because you said no before. Do what feels right to you at the moment and don't worry too much about the future or the past -- the one you can't change and the other you can't predict.
One more thing -- never have sex without a condom unless you want to be pregnant and/or infected with sexually transmitted diseases. Even if he promises you that he's a virgin, he most likely isn't. Of course, hat doesn't necessarily mean that he's trying to cheat you; he might just be embarrased about it and doesn't want to tell you, maybe for fear of losing you. You can't expect a sixteen-year-old boy to be completely accurate with you because the truth is that they have a hard time being accurate with themselves.
