The device, thought to be a large firework with nails buried in it, went off inside a teacher's car only minutes before the end of the school day.
No-one was injured in the blast outside Runnymede St Edward's school in Sandfield Park, West Derby, Liverpool at around 2.50pm on Friday.
Superintendent Ian Pilling from Merseyside Police said: "Certainly from what I have seen anybody in the vicinity of the vehicle could have been injured or killed."
Mr Pilling said: "This was a despicable offence. Children were in the process of leaving. Nobody was killed or injured but that was down to good luck."
The car, a silver saloon, was parked 30ft from the windows of the school, which teaches children as young as three. Fragments of nails and shattered glass from the windows of the car were sent flying into the air when the device detonated.
Around 300 pupils were still in their classrooms awaiting the end of the school day at 3pm. They were kept inside as police checked the car before being evacuated later.
Forensic officers were examining the scene of the blast, alongside CCTV from the school's cameras. Officers said they did not know whether the teacher who owned the car had been targeted directly.
Mr Pilling said: "This is an extremely unusual offence and we are investigating fully to bring the offenders to justice as quickly as possible. At this stage we can't say if somebody was targeting a teacher, there is nothing indicating why anybody would target this teacher."
He urged parents collecting their children, or members of the public who might have seen anything suspicious, to contact officers. The �4,500-a-year Catholic school for children aged between three and 11 is set in 30 acres in a suburb of Liverpool. It is the choir school to the city's Metropolitan cathedral.
Last Mod: 22 Eylül 2007, 16:49