You may have issues with an 18th, lots of places don't like them.
I would suggest hiring a security guard, let the hall people know and that might help.
Inform the local Police, ours responded very quickly when we had a bunch of crashers trying to get in. They then did a friendly walk through, just to let the party goers know they were aware of it.
At the end of the day we had no issues, Police and security didn't man up but just dealt with the kids with respect and all went well.
Bunyip