They were slain at Bukoh school in tambon Prachan about 11.45 am, said Pol Lt Wicha Noopaennoi, deputy investigation chief at Yarang police station.
The four defence volunteers were assigned to provide protection for teachers and students.
They were sitting under a raised school building when armed men dressed like soldiers arrived on motorcycles. They pretended to inspect the school, greeted the security detail, and then opened fire on them.
The gunmen then fled with the slain men's four AK rifles, Pol Lt Wicha said. They scattered metal spikes over the road behind them as they left.
The four dead volunteers were Sulaiman Wae-useng, 36, Muhammad Taeteng, 33, Abdulloh Salae, 35, and Bueraheng Ji, 32.
Explosive ordnance disposal officers were dispatched to scour the school grounds and surrounding areas for bombs.
Police investigators reported there were at least eight attackers. There was evidence the two sides had exchanged fire. Some of the gunmen might have been hit.
A massive hunt was launched for the assailants.
A teacher said students were studying in class on the floor above. Kindergarten pupils were having lunch in rooms on the first floor near the scene of the attack.
The shooting frightened students and teachers, whose morale was severely shaken, the teacher said. All students were quickly picked up by their parents after the attack.
Col Pramote Prom-in, spokesman for the Internal Security Operations Command Region 4's Forward Command said the gunmen were dressed like paramilitary rangers.
He expressed his condolences to the families of the slain volunteers, who had performed their duty well, protecting people until their lives were ended. Their families would receive welfare benefits.
The attack showed the assailants were willing to violently attack people without choosing specific targets, he said.
Earlier, a string of bomb blasts wreaked havoc in Songkhla province.
Two bombs damaged two high-voltage power poles in Khuan Niang district and four more explosives, attached to another four power poles in nearby Bang Klam district, were found on Dec 26, following the bomb blasts on Samila beach in Muang district of Songkhla.
On Jan 8, a car bomb went off in Thepa district of the southern province, destroying a power transformer and injuring two policemen. The explosive was packed in two 15kg gas cylinders in the back of a pickup truck stolen from a retired teacher, Amata Samonthanthawee, 62. Amata was hanged by insurgents at his home in Saba Yoi district before the truck was used for the car bomb.
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