Consider the standards of behaviour you expect and stand firm about these expectations. More parents and young people are keen to support you than you might expect.
Consider contacting the parents of students who are extremely drunk.
Consider how to deal with situation where a student is behaving badly (eg call parents, isolate from main party, call police).
If uncontrollable situation develops don't hesitate to call the police.
If student is unwell - drunk, vomiting, etc. - consider contacting parents. If parents are unavailable, have contingency plans eg put in separate room and observe student constantly or ring for an ambulance or medical assitance. Excessive alcohol consumption by teenagers who are young or inexperienced can pose great risk.
Be aware of the ramifications of allowing inebriated teenagers to leave your premises without the supervision of a responsible adult.