Parents of 170 pupils at the independent school - more than 100 of them boarders - were told on Wednesday. A spokeswoman said the the decision was taken following a risk assessment of the potential disruptions associated with the conference.
The school is situated next door to Crieff Hydro where the American delegation will be based. It is understood that police had warned of the potential for demonstrations.
Ardvreck, which takes children aged three to 13, was due to close on July 10 but will now close on July 2.