A picture of a girl holding a beer somehow was placed in the school yearbook in Illinois.
The faculty had no idea the picture was included until yearbooks were handed out to students this week, the Chicago Tribune reported Friday.
"It's clearly defiant and subversive and intentional," says spokeswoman Laura Blair.
Principal Linda Yonke did not consider the prank harmless.
"It sort of casts a pall over the whole yearbook," Yonke said.
The snapshot in question is of two girls hugging each other while one of the girls holds a beer can behind the other girl's back.
School officials say they'll take appropriate disciplinary action against those responsible for the prank, the Arlington Heights Daily Herald reports.
New Trier has a long history of senior pranks, says a New Trier alumni.
One year keys to the school were mailed out, forcing officials to change all the locks. In the 1960s, seniors disassembled a car and reassembled it in the first floor rotunda.
That's a prank, but not a good one. They should have been more inventive, and the yearbook instructor should have checked the pictures better.