Thousands of nails lay scattered on the bus ramp. Cords stretched between poles in front of the school. And North Harford High School's front door was glued shut.
A message - "07" - was written in caulk on a wall near the school library.
"Whether it really was the Class of 2007 or someone trying to implicate the 2007 class, we don't know," said Don Morrison, Harford County schools spokesman.
School officials and county sheriff's deputies are investigating what they described as an end-of-the-school-year prank. Deputies were notified of the nails about 4 a.m. yesterday.
Morrison said the incident did not disrupt the school schedule. But it snarled traffic as parents dropped off their children and buses were rerouted, said sheriff's deputies.
As a precaution, buses dropped off high school students across the street at North Harford Middle, which also had cords wrapped around the front area of the school but no nails, according to sheriffs deputies.
The areas at both schools were cleaned by the time students arrived, but the high school bus ramp area was cordoned off because of fears of stray nails, Morrison said.