The simple answer is that whatever you wear is inconsequential. If you look under 18, no amount of clothing or makeup will get you through the nightclub doors.
The only thing you can do is go to pubs/clubs where you know they might be a little "lax" or be in a party of people who are over 18, the chances of you slipping in un-noticed amongst them are better.
As a mum, I really should be telling you about you being underage and it's dangerous etc etc etc, but you're 17 for heavens sake, it's what most young folk (corrr i sound old eh?) do. Just be extremely careful and don't do owt too daft or anything which puts yourself in danger.
Good luck :-)