Denville police say the students - all boys, whose names were not released because they're juveniles - were placing rabbits, mice, roosters and chickens inside ceilings at Morris Knolls High School when they were caught this week.
Officers went there shortly before midnight Tuesday after a custodian reported seeing people inside the building. Three students were arrested there, including one who allegedly was trying to steal a school bench, and the other four were taken into custody over the next few hours.
They now face various charges including burglary, criminal mischief and conspiracy. And authorities said Saturday that animal cruelty charges are pending because at least one of the animals was injured after falling through the ceiling.
Police Lt. Paul Nigro said the boys gained access to the building through a window that had deliberately been left open earlier in the day. He said they admitted stealing most of the animals from farms in the Morris County area, but legally bought the mice.
Morris Hills Regional Schools Superintendent James Jencarelli told the Daily Record of Parsippany that all seven boys were barred from Friday night' prom and may face additional disciplinary action at school.
It's the second straight year that an animal-related prank has occurred at the school. Officials say unknown conspirators put a dead deer - which apparently had been killed by a car and retrieved from a local roadway - inside a girls bathroom last year.