"Throughout the Troubles politics was kept out of schools, but that has changed." The views of the head of one of the largest teaching unions in the north of Ireland were echoed by every teacher who spoke to Róisín Ingle last week. Death threats against Catholic teachers - threats which were subsequently lifted, but caused great distress - and more harrowing scenes at Holy Cross Primary have left the Northern education system looking more vulnerable than ever.

From the outside, Our Lady of Mercy school in north Belfast looks more like a fortress than a place of learning. The front gates are firmly locked, only grinding slowly open when the visitor is identified via an intercom. Two big brother-style cameras track your path to the front door of the school building. There is a police Land Rover parked in front of the school, razor wire on the roof, high security-fencing around the perimeter and metal grills on every window.

A cleaning company is busy sweeping away the last traces of debris from the incident, less than two weeks ago, where six loyalist men drove into the school and smashed up 19 teachers' cars. They wore balaclavas and wielded sledgehammers, crowbars and guns. Frightened first-years watched the scene unfold through a classroom window. The lesson that day, caught on close circuit television, was naked sectarianism.

The school is located in the heart of loyalist Ballysillan, just 500 yards from Holy Cross school, which had been closed on the day of the attack. Headteacher Peter Daley outlines what this has come to mean for the 500 students at the school. "They can't walk to or from school - they have to be bused in. They can't go on to the playground, they can't walk to the nearby leisure centre, they can't eat their lunch outside," he says.

What the location of the school has meant for the teaching staff over the years - particularly since developments at nearby Holy Cross School in Ardoyne - is that education has to take second place to crisis management.

Some £100,000 sterling worth of damage was caused when the school's resource centre was petrol-bombed during the summer. Security features worth £300,000 sterling have been installed.

Daley, who was appointed principal in September after a 25-year teaching career at the school, has kept a log of violent incidents and intimidation against the school. It is filled with report of injuries sustained by pupils on buses that are regularly stoned or sectarian abuse hurled by adults standing menacingly at the school gates.

After the cars were smashed, teachers in Catholic schools across Northern Ireland learned of a death threat placed on them by a loyalist paramilitary group. The killing of 20-year-old postal worker Danny McColgan a few days later did little to allay their fears.

"What happened that day with the cars has caused a lot of stress to the teaching staff and when we heard about the death threat, the pressure obviously grew. I phoned each teacher over the weekend to assure them that if they didn't feel happy to come to school that would be understood. On Monday, every one of our teachers turned up for work, except one who is off with flu," he says. There was virtually full attendance among the student body too.

"Our chief concern at the moment is offering an oasis of calm to the children. When they walk in the door, they can forget what is going on outside and worry about things like whether they have done their homework," says Daley. "We have to keep going, keep things normal so that they don't have too much time to dwell on it."

Some of his teaching staff, sipping coffee in a staffroom agree, while admitting that the stress is taking its toll. "There wasn't really a decision to make, you had to come back to school. But you are more nervous, some of us have changed our route to school, for example, and I am more observant. I find when I come home from school I am exhausted because of all the adrenalin during the day," said one.

A new teacher describes the situation as "very scary, very stressful". Another was filling out special consideration forms for some of his pupils who had sat exams the day after the incident. "There is no doubt it would have affected their performance," he said.

This targeting of schools is not just confined to turbulent north Belfast.

A school on the leafy Malone Road in the south of the city received threats, while there was an arson attack on a school in Lisburn. And it wasn't just Catholic schools. The Protestant Boys Model School in north Belfast had to be evacuated after a bomb scare. All over Northern Ireland, the teaching profession is on tenterhooks, nerves frayed by the utter unpredictability and the callousness of the attacks.

Tom McKee of the National Association of School Masters and Women Teachers, the largest teachers' union in the North drawing members from both side of the sectarian divide, says the team spirit of teachers has been remarkable.

"There has hardly been a single teacher absent during all of this, even though there are reports that some are having to take tranquillisers to get them through it," he said. "Recent events are a very disturbing escalation. Throughout the Troubles politics was kept out of schools, but that has changed."

While much of the focus has been on Catholic schools - the death threat against Catholic teachers was lifted last week - Protestant schools in North Belfast have their own problems as riots raged nightly outside the school gates.

"The hardest thing is to maintain a sense of normalcy," said Johnny Graham, principal of the Girls Model School in Ballysillan. He has noticed an increase in text-messaging among the students as news of the latest bomb scare or arson attack is spread by mobile phone.

Like most schools in flashpoint areas, there is a comprehensive programme to encourage "mutual respect and understanding" of other communities. But, says Graham, it is impossible to control what goes on when students leave and go back into their segregated lives.

The buses taking his students home from school are stoned almost daily.

"There is no excuse for kids from either side being hassled in this way. What is really sad is that our students have to do A-Levels and are in competition with students in places where there is no trouble at all," he said.

This is a source of frustration too for Jim Keith, principal at the Belfast Boys Model School in Ballysillan. "This is a deprived area," he says. "So, more than anywhere, young people need their education and the support of the community - but, unfortunately, education has been taking a back seat."

His time recently has been taken up with procedural issues, how to reduce the time it takes to evacuate the school in the event of a bomb scare or communication with police after students were taken home under armed guard for their own protection. He worries that recent events will stop good teachers wanting to come to work in north Belfast.

"They used to say schools were sacrosanct before, but not now. You just don't know what is round the corner and so most decisions at the moment are being taken on the hoof," he said. "I was very depressed for a few days, but with the lifting of the threat, I am a little more hopeful."

The sheer resilience of those in the Northern Ireland education sector, not forgetting the cleaners who are alone in schools from early morning to the caretakers who lock up at night, has never been so visible.

At Our Lady of Mercy, drama teacher Patricia Meir, herself a past pupil at a time when the school endured several bomb attacks, recalls shepherding her students away from the window as they witnessed the destruction of teachers' cars.

"It was the mindlessness of it that struck me most," she said. "But straight away we realised that our priority was the safety of everyone in the school."

She speaks with confidence about the future for pupils and teachers at Our Lady of Mercy.

"The students know it is our right and their right to give and receive an education in a safe and secure environment. The incident has not changed that. The students came back to school and so did the teachers, which was wonderful. When I was here as a pupil, the nuns always made us feel proud of our school. There is nothing defiant about it, we just know we are doing the right thing."