An explosive device fell on top of a workman cutting a hedge in school grounds in Co Antrim yesterday.
As workers fled during the alert at Harryville Primary School, Ballymena, the alarm was raised and terrified children were forced to vacate their classrooms for the second time in a month.
The device was discovered around 9am and police ruled out suggestions it was part of a cache of loyalist pipe-bombs that had been hidden beside the school in September.
A PSNI spokesman said they believed the new device had only recently been left there.
The local MP, DUP leader Rev Ian Paisley, said the security forces needed to sweep the area thoroughly to ensure there were no more devices.
Last month's pipe-bomb cache was left in the area on September 12th during loyalist rioting in the wake of the Whiterock parade.
School principal Lesley Meikle said: "The [ school] board maintenance men were here this morning. They were cutting the hedges for me and it fell out on one of the men that were cutting.
"Thankfully it didn't go off. That was a blessing. It happened around 9 o'clock this morning when we were taking children on the way to swimming. The maintenance men were rather shaken and they are away off."
Mrs Meikle said 142 pupils were evacuated and although last month's evacuation went smoothly, she said there was "a bit more panic today because we put in procedures from the last time that . . . a different bell would sound if we had to go the opposite way from our normal fire exit. So as soon as that bell rang the older ones knew there was something up . . . There were some tears."