The leader of Nigeria's Islamist militant group Boko Haram has called for more attacks against schools, describing western education as a "plot against Islam", in a video released days after his fighters killed 46 students in an assault on a dorm.

In the 15-minute recording released at the weekend, Abubakar Shekau said schools would continue to be targeted "until our last breath".

"Teachers who teach western education? We will kill them! We will kill them in front of their students, and tell the students to henceforth study the Qur'an," he said, gesticulating energetically while dressed in military fatigues and a traditional hat.

Shekau denied that his fighters killed children. "Our religion does not permit us to touch small children and women, we don't kill children," he said, reading from sheets of paper as he cradled a Kalashnikov. He also dismissed talk of a ceasefire. Last week the government said it had signed a deal with Shekau's second-in-command.

With its ability to launch attacks reduced by a military crackdown, Boko Haram is redrawing the battle lines in Nigeria's four-year insurgency by going after softer targets. A recent spate of attacks on schools is part of a two-pronged strategy that plays up the extremists' ideology against western institutions while also providing a stream of potential new recruits as frightened parents pull their children out of education.

Unschooled and unemployed children are increasingly being recruited – sometimes forcibly – to fill the ranks of Boko Haram and unleash violence against their peers, the Guardian has learned. Witnesses say many are plied with dates stuffed with tramadol – a narcotic used to tranquilise horses – before being sent on missions.

Just after dawn on 6 July, a school dormitory was doused in petrol and set alight in north-eastern Yobe. Those trying to flee the flames were shot. The attack left 46 dead, mostly students. More than 300 classrooms have been torched in the remote, arid state since 2009, according to official counts.

Hundreds of families have fled the region. "This really shook us up. Students being attacked in their sleep is too disgusting for us to even imagine," said Adam Mohammed, a textiles trader visiting neighbouring northern Kano state, where he relocated his family for safety reasons. "It was hard, but I feel I made the right decision to leave Yobe. I'm a father of three and when I think of what those parents must be going through …" He shook his head mutely.

Last month 16 students were gunned down in consecutive strikes on a secondary comprehensive in Yobe and another school in Borno, Boko Haram's spiritual home. In April two attacks on a university left 16 dead.

A state of emergency has been in place for two months in three north-eastern states. Soldiers pouring into Yobe and Borno have dismantled urban cells, but Boko Haram has responded by changing tactics. Previously it had attempted to ignite a sectarian war by bombing packed churches.

Closing down mobile phone services is thought to have reduced the insurgents' ability to co-ordinate attacks, but it has also had unintended deadly consequences. "I saw the gunmen sneaking into the school compound in hordes but I couldn't call any soldiers for help," said Ahmadu Sani, whose farm borders the school grounds where 46 were killed. "The police cannot be everywhere so they should restore the connections even if there's a state of emergency."

Northern Nigeria has some of the highest unemployment and school dropout rates in the world, despite the country being the eight largest oil exporter. At one of the many checkpoints around the Yobe capital of Damaturu, a soldier said the number of children hawking on the streets had ballooned in recent weeks.

"As they are no longer in school, their parents send them to sell groundnuts or boiled eggs on the long queues of vehicles created by the stop-and-search," said the soldier, who like others had removed his name tag from his uniform. "We know they are paid by the Boko Haram to spy on us."

Mohammed, a gardener working in the economic capital of Lagos, said he had fled from his village of Dikwa, a few miles from a large Boko Haram camp. "The Boko Haram were everywhere. They collected taxes from us. They stripped one Muslim girl naked and beat her because they said she didn't cover her ankles," he said, looking nervous at the mention of the militants.

He said two men had turned up at his grass-roofed house in May. "They said the almajari [religious school] my son was going to was haram [forbidden] because the imam used prayer beads. They gave me all kinds of warnings. They said that I shouldn't cross my arms when I was talking to them because crossing the arms is haram too."

The final straw had come days later when his family were awakened by a neighbour's wailing. "[Boko Haram] told her they took her son to their camp to fight for Allah," Mohammed said. "They said the boy's family is now the Boko Haram. My wife said we should leave that very day."

At the defence ministry in the capital, Abuja, a senior official showed footage found on the mobile phones of alleged Boko Haram members. In one, a suicide bomber barely old enough to be out of primary school showed off his new sunglasses and joked in rapid slang with fellow teenage members as he got behind the wheel of a car packed with explosives. A few minutes later, his friends filmed the car blowing up outside an army barracks.

"Our structure has never been geared towards the current challenges – suicide attacks, IED attacks. These are tactics that until very recently we only saw on television, just like the US was rudely awakened by planes entering into buildings," the official said. "It's not just about training Nigerians how to shoot. We need to look at what terrorism will look like in 20 years from now."

Vigilante groups armed with sticks and machetes prowl the streets of Borno and Yobe, complicating efforts to flush out the insurgents. "They want to help out but they're also a nuisance," said a soldier based in Borno. "They're not professionals and they're not trained for a job like this. They're too many of them and it's hard to manage them."

"The problem is some of them have lost their loved ones during the course of the insurgency and they're looking at vigilantism as a way to get revenge. The bad guys have been pushed out of the towns. They're resorting to ambushes from the villages now, and that is a different kind of warfare."

Issac Abrak contributed reporting from Yobe state