Some BS grad parties do happen across the Canadian border. Another benefit is if its in an urban area like Montreal or Quebec City, it minimizes the potential for drinking and driving.
I think pretty much every boarding school sends out those letters/emails prior to graduation and even some long weekend breaks. Parties still happen and parents obviously are ok with it.
@SoapyMango - Perhaps your child and his/her friends are some of the kids who choose not to party at BS but I can assure you it goes on at Andover as it does elsewhere.