If it makes you feel any better, I have been in a very similar situation.
I hate clubbing, I don't drink a lot, I have never been drunk, and I don't plan on getting drunk. Oh, and I'm a 19 (nearly 20) year old girl.
If it makes you feel uncomfortable, don't go. When you're 18, people will start having birthday parties in clubs, which is a good excuse to "try" it and see if you like it.
I still hated it, and now don't go out clubbing, because I know if I go, I will just sit at the side trying not to bang my head on anything in a desperate attenmpt to go home!
If you don't enjoy it, then you'll be miserable (and so raise suspision lightly if you're underage) and the others are unlikely to worry about you after a certain point.
Why not suggest going out somewhere else (like the pub when you're 18). Your friends can then go on to places aftyerwards and you don't feel like you haven't spent time with them.
Or, if you do go clubbing - don't drink excessively and use the excuse that one of you needs to be sober to help everyone out when they are! Or when you can drive, volunteer to be the designated driver. That's what I've done!
Hope that you don't feel so isolated!