Drones and missiles that targeted oil facilities were fired from southern Iran, a US official says.
The banner was held up during an American football game but the school bars political endorsements.
The prime minister's remarks come after the arrest of senior RCMP intelligence official last week.
One of the attacks was on an election rally where President Ashraf Ghani was due to speak.
Javier Burillo is accused of being drunk while operating a boat that hit and killed his 11-year-old.