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I would Say have a small Party for your 18th Birthday with a few close friends and Family. And since it's Grad there will be a ton of Grad Parties. Just get involved with those if it means that much to have a Grad Party, My Schools bigger but student council somehow is planning a party every weekend. Which seems a bit much. Just Get involved with the other Grads and go to there planned parties. Participate in all the Grad Activities and see where it leads.

Both are important events, and you should only invite important people, like family and good friends. Those are the people who will appreciate the fact that you are a high school graduate or are legal to do more than just drive smile
Believe me, I will be graduating/turning 18 next year as well, but I am graduating from a much bigger school (like 350-450 in the graduating class alone) so the graduation is a much smaller issue, not only since there are a bunch of people graduating but I'll probably be going to Abby's pizza with the band after graduation xD
And I never make a big affair out of my birthday because it's a hassle. I much prefer celebrating with good friends and my parents rather than have a gigantic party that I have to provide food/entertainment for just for the benefit of a few more presents. It's much more fun that way.

Basically, if they're not your friend, they won't appreciate you or your celebration and that's no fun for anyone.

I never invited many people to my parties, because I didn't know many. It was only in high school that I had an idea who I actually wanted to invite. Most of my parties consisted of eating around family and friends (and not many friends besides - only about five!). I think all a celebration really needs is people who care enough to celebrate with you. Having fun with a few very close friends is better than having a bunch of people around who you barely know.