We have a time-honored tradition in the US called the power hour. Once upon a time, I was in a fraternity, and this may or may not have been a hazing thing, too... but it's also popular as a "congratulations, you can legally drink!" celebration. And it's a lot safer at 18 than 21...
So the way the power hour works is you go to the bar. And you have one hour to drink 18 shots (as opposed to 21 in the US.) It can be whatever you want, although I recommend relatively low proof straight shots... well rum is a good bet.
As for not all your friends being 18... just gotta be smooth at the door. I used to go to the bars all the time underage. The secret to being places you aren't supposed to be is to act like you're supposed to be there. When the waitress asks what you want, you order a pint, you don't take a minute to think about it or look at the list, and you don't order a fucking imperial pint-- I was being dumb the first time I snuck into a bar, and I was like "Hey can I see the list? How about an imperial pint?" If you don't know what you want and you try to be fancy (yes, they did sell imperial pints at this bar, I wasn't just pulling that out of my ass) you're going to get carded. If you say "Pint'o Guiness" and turn back to the TV in the bar, nobody'll ever say anything.
IN THE US? ASK ME HOW TO GET 10% OFF GAMES WORKSHOP PRODUCTS!