The culturally distinct region that borders Malaysia has seen rebels fight against the rule of Buddhist-majority Thailand, which annexed the area over a hundred years ago.
Since 2004, armed insurgents have clashed with Thailand's powerful military, killing nearly 7,000 people - mostly civilians - both Muslims and Buddhists.
The death toll dropped to a record low last year as Thailand's junta tightened its security web in the south.
But recent weeks have seen an uptick in violence, as rebels show they remain able to carry out operations after 15 years of conflict.
The men killed on Thursday were all Muslim and were guarding a school in Yarang district of Pattani province when the gunmen struck.
An AFP photographer at the scene saw uniformed bodies on a blood-splattered floor in the school compound.
"They were shot dead at the scene," Lieutenant Wicha Nupannoi, of Yarang police station told AFP, adding the killers were suspected insurgents.
"The assailants seized their four rifles," he said, adding there were no eyewitnesses to the attack, which took place before lunch.
Civil defence "volunteers" are poorly paid local civilians - both Muslim and Buddhist - who are lightly trained, armed and paid by the Thai state to supplement security forces who have blanketed the southernmost provinces throughout the rebellion.
The 15-year insurgency has seen scores of teachers killed, slain for their perceived collaboration with the Thai state, and led to the deployment of armed guards at schools.
In a rare public statement dated January 4, the main rebel group - the Barisan Revolusi Nasional (BRN) - which has command and control over most of the rebel foot soldiers, swore to "keep fighting."
"Siam (Thailand) can't hold out," the BRN wrote, signing off with a warning: "Do not help and support Siam."
Prolonged peace talks between the Thai government and an umbrella group which claims to represent the rebels have failed to yield peace.
But Thai authorities as well as the Malaysian facilitator of the talks have recently expressed confidence they will reach a detente soon.