Brendan Shannon, from St Comgall's Primary on the Ballymena Road, lifted the pipe bomb and handed it over to one of his teachers, who alerted the PSNI.
The school's 400 pupils were then moved to a nearby church hall shortly before 8.30am on Monday, before being sent home.
The P5 boy and his twin sister, Ciara, had arrived early on their bicycles to help deliver milk to classrooms when he noticed the device lying on top of a painted line close to the playground wall and went to find the caretaker.
"I saw a golden pipe thing that looked like a golden pipe bomb", Brendan told UTV.
"I just got off my bike and touched it, then I was okay, so I lifted it to try and find him but I couldn't find him but I found two teachers and told her where it was."
The device was declared viable by army bomb experts. It has now been removed for further forensic examinations.
Brendan's father told UTV: "The reality hasn't sunk in."
"I'm certainly extremely worried and trying not to think about what the consequences could have been", Gerard Shannon said.
"It could have been horrific to think that he left home and within five to ten minutes who knows what could have happened?"
A search at a second Catholic school - St Joseph's on the Greystone Road - began shortly after 10.30am when a phone warning was received by a local media outlet. Pupils, including a nursery section, were also evacuated.
Nothing untoward was found at St Joseph's school.
Justice Minister and South Antrim Alliance representative David Ford told UTV a loyalist splinter group "appeared" to be behind the two security alerts.
"It appears we have a small group of people in and around the town calling themselves the Real UFF who set three pipe bombs on individual houses a couple of weeks ago, and now it appears to be the same group who both attacked children at St Comgall's and also threatened children across the town in putting another threat, although there was nothing behind it", Mr Ford said.
"It's something absolutely horrific. It doesn't matter who is doing it, what causes they're claiming. It has to be condemned by everyone".
St Comgall's school is not far from Antrim police station.
"I cannot express enough my disgust at the cowards involved in these alerts today," Chief Inspector Simon Walls, area commander for the district, said.
"To target the general public is never acceptable by any means, but to take away the secure feelings of innocent children and to put them at risk like this is beyond despicable.
"It is by sheer good fortune that we are not dealing with a severely injured child right now."
Sinn Féin Education Minister Caitríona Ruane said: "Whoever caused these alerts must be strongly condemned for putting lives at risk.
"At the start of a new school year, when some children are attending for the first time, this has caused unnecessary fear and disruption in settings which should give a sense of safety to the children and all school staff.
"I would ask anyone with information about these incidents to pass it on to the PSNI."
DUP MP for South Antrim William McCrea said those responsible had "plumbed new depths".
"It is obscene that any person would leave an explosive device anywhere, but to plant a bomb at a primary school is utterly disgraceful.
"To target a primary school and to put innocent children at risk is plumbing new depths", he added.
Ulster Unionist South Antrim MLA Danny Kinahan said the alerts were "disgusting and utterly unjustifiable".
"Those who would plant a bomb or even orchestrate a hoax at a school are beneath contempt."
Local SDLP MLA Thomas Burns also condemned those behind the incidents.
"There is obviously a group which is determined to cause mayhem in our town with these regular pipe bomb incidents, and until we know otherwise we have to operate on the basis that they are prepared to cause murder as well," the South Antrim MLA said.
"There is only one place for the sort of people who left this device and that is in jail for a long time."