On Tuesday, seven schools received bomb threats. Similar incidents also occurred in other parts of the UK.
Police said there is nothing to suggest the incidents are terrorist-related.
"We continue to investigate who is responsible and whether these incidents are linked to similar calls made to seven schools earlier this week," Ch Supt Garry Eaton said on Friday.
Among the schools affected on Friday was Kilmaine Primary School in Bangor, County Down.
Ulster Unionist MLA Alan Chambers, whose grandchildren attend the school, said 600 children were moved from it to a nearby church.
"Fortunately, the pupils seemed to be taking events very much in their stride and did not seem to be distressed in any way, even though they had to leave the school without their coats or lunch boxes," he said.
"It was clear that the parents and grandparents who arrived to collect their children were upset at what had happened.
"I simply cannot conceive of the mentality of anyone who would target young children in his manner.
"The number of alerts that have been received this week indicate a degree of sophistication and organisation. It is imperative that the police act swiftly to catch whoever is responsible for this disruption."
Kilmaine PS principal Billy Campbell said they were interviewing for teaching jobs when the secretary came in said they had received a bomb threat.
"We immediately took the decision to evacuate the school. We had already thought about this beforehand because, in light of these other incidents happening at other schools, we thought 'what will we do if it happens here?'" he said
"So we had a plan in place already, which we hopefully weren't going to have to use, but it was there so we did it today.
"As soon as the message was received, the building was evacuated very professionally by all the teachers and classroom assistants, the children were all fantastically well behaved as I knew they would be."
Other schools threatened on Friday included Armstrong Primary School in Armagh and Omagh County Primary School.
Four schools in Northern Ireland have received bomb threats on Friday, three days after others received hoax  calls.