King's College went into "lockdown" this week with sniffer dogs searching the school boarding houses.
A concerned parent, of two boys at the school, told the Herald on Sunday parents raised concerns at a meeting on Tuesday night.
"Out of that came the decision to have the school go into lockdown mode on Wednesday morning. Dogs were sent in to search every inch of the school. Parents had no notification," said the parent, who asked not to be named.
"Parents are paying enormous fees and you cannot get anything out of the school - you have to rely on your children for information," she said.
Seventeen-year-old David Gaynor died after an incident at a motorway overbridge following the school ball earlier this year and 16-year-old James Webster died in 2010 from acute alcohol poisoning after drinking a bottle of vodka at a party.
Headmaster Bradley Fenner said the searches were part of the school's firm line.
"Last week we had a couple of sniffer dogs come in and go right through the school. We had told parents beforehand in a letter.
"We have had a lot of positive feedback from parents who are delighted with our firm line," he said.
"As a result two students left us because of their involvement with cannabis and that is all the information I can give you."By Joanne Carroll Email Joanne