Police were called to Ancaster High School on Monday after staff saw what they thought might be someone with a weapon on the school's roof.
The staff member saw a young person on the roof with a “long object protruding from a backpack,” Hamilton police say.
Officers went to the high school around 4:35 p.m. on Monday, securing the school as they investigated. But the youth wasn’t there anymore.
They found a World Cup flag on the roof.
Police are considering the matter closed.