If you and your teenager are hosting their party, sit down and agree to some rules. Adult supervision is critical at teenage parties. Make sure it is visible but not intrusive. For example, the teenagers could be in the garage while the adults are in the lounge. You may ask that all the guests come through the front door first so that you know who is there.
Gatecrashers can be a problem.
To prevent this happening, some parents hire security staff or ask a bulky adult relative to deal with uninvited guests. Set a finish time for the party, and have a group of adults work together to wind it up - this can involve other parents coming to pick up their own kids. One parent said she clears up empty and full cans as soon as the party is officially over. This works every time.