Underage Drinnking? What could happen?

Okayy so a friend of mine will be turning 16 soon (I'll already be 16 as will half the people there though some will be 15 still and a few 17) and she wants to have a themed party at our local football clubhouse. It will be a big event with possibly 40 to 50 teenagers there. It would be supervised with bouncers at the door and Im pretty sure her mum and/or parents will also be there. No one is allowed in unless you have an invitation and your not allowed until unless your lift home is waiting for you outside. There will be alcohol allowed there (although Im not sure if we have to bring our own or if it will be supplied. Would this make a difference?) and Im wondering what would happen if the police came? Or do you think the police would even come in the first place? There will be a DJ so lots of loud music but there isnt really too many houses close by, so I doubt someone would ring the police for the noise level. If the police did come what would happen to the people drinking? and the people not drinking? Would parents be contacted? Thanks heaps :) x