Although I agree with almost all of the answers you have been given here, there is one thing I noted that you said... You are ALMOST 18. That means you are both underage and legally you are in the clear. However, when you turn 18, you can be charged with a sex crime if you continue with this younger person. You need to keep that in mind. It won't matter that you're only 2 grades apart at that point. Once you turn 18, you are legally an adult and law enforcement will see it that way. That addresses the legal side of the issue.
The bigger issue is your manipulation of the situation. I read where you stated that you are grown up, but as long as you treat people this way, I disagree. When you are mature, you don't use people in this way. Relationships are two-sided and you need to learn to treat other people with respect. A mature relationship does not use lies and manipulation in order to get what you want. A mature relationship relies on respect, trust, caring, and understanding.
Bottom line... this guy is very young. You know it and we know it. He will mature in time and when he does he will realize just how wrong you are and he will resent you for it.
Since the whole situation seems to be about sex for you, you need to find someone older who just wants sex and leave it at that. I am not advising that, because personally I think you need to mature before you continue with any relationship... but if it's sex you want, go for it. Just do it with someone old enough to understand that that's what it is.
In the meantime, stop making excuses for your pitiful behavior and accept that what you did is wrong. Very wrong. You can't change bad behavior until you own it.