We’re perched 3,000 metres up the Jbel Sahro mountains of the Moroccan Atlas, seven weeks after leaving Liverpool. It’s the middle of August. As the 25 students spread out their sleeping gear and prepare to cook the evening meal, lightning begins to flicker around the mountain pass and thunder rumbles. A couple of shepherds who are helping us to clear scrub tell us it won’t rain, but quite suddenly a fierce gust of wind scatters belongings, heavy drops of rain begin to fall and moments later a torrential downpour sends everyone leaping for cover into the trucks. Leaving bags where they lie, we hug ourselves against the freezing cold. “It’s not supposed to hail in the Sahara,” wails someone. Of course, he’s right, but we’re half-way through an expedition which has produced surprises all the way.

Over the past 25 years, Shorefields Community School in inner-city Toxteth has notched up a pretty incredible achievement. It has sent no fewer than 12 expeditions overland to the Sahara. The usual route is through France and Spain into Algeria, sometimes getting as far as Mali or Niger. This time, due to political unrest in Algeria, the planned route is through Morocco into Mauritania.

The expeditions were started in 1970 by Alasdair Kennedy, head of geography, who’d visited the Sahara while at university. He joined the staff of Shorefields 28 years ago and has run outdoor activities for young people all over north-west England.

But why choose the Sahara for a school expedition? “It’s the last great wilderness within easy reach,” says Al Kennedy, “where you can travel for days hardly seeing a soul and put your survival instincts to the test. There’s something very challenging about the heat, the quiet, the great emptiness - the total contrast to life in Liverpool.”

Previous expedition members have returned from the desert with a more mature outlook on life to see their home city in a new light. Some have gained the confidence to go travelling round the world; some have gone in for jobs they wouldn’t have thought possible before; some have become expedition leaders. Two of the leaders on this trip, Pip and Lucy, met on the 1982 expedition, the first one in which girls took part. Lucy is now a dentist and the expedition medical officer; Pip works for the university accommodation service in London and is expedition treasurer. The school itself raises money to pay for the l0-week trip (no help from the City of Liverpool) which lasts from mid-June to September; the students contribute Pounds 360 each plus their spending money.

Other staff have played a key role over the years. Kevin Barber is expedition mechanic; he used to run a car repair business before going into teaching. Alan Martindale, PE teacher, is quartermaster and is responsible for ordering and loading the three tonnes of food and supplies which include everything from toilet rolls to heavy welding gear.

The school owns three ex-army Bedford lorries, which have been customised to carry equipment and water, and the students do a lot of the work in preparing them for the journeys, There have been near-disasters in the past; the trucks were badly vandalised one winter and a small piece of grit in an engine led to a breakdown with almost fatal consequences in the Sahara. On another journey they found a group of travellers, illegal immigrants, and rescued them from almost certain death after they’d become stuck in the sand.

The students, aged from 15 to 18, come from a variety of backgrounds. Some have parents who are employed, some live with a single parent or with grandparents, many live in the troubled area of Toxteth where unemployment is endemic. On the last trip there were fights and a boy had to be flown home, but on this occasion there is a quiet sense of working together to share the daily grind of keeping the expedition fed, watered and on the move.

After spending a few days to acclimatise on the Moroccan coast, the expedition heads inland to the Atlas mountains. This is the first test for the 20-year-old vehicles as they leave the tarmac and take to mountainous tracks built for horse-drawn transport. A puncture causes a loud explosion; a rear tyre ripped open gives the assistant mechanics their first opportunity to get into action. As the two trucks struggle slowly up the mountain there are a series of alerts; everyone has to pile out to rebuild sections of the route where it has crumbled away. On one occasion a wheel lunges over a steep drop and the occupants jump out so fast there’s no time to get shoes on.

The scenery in the mountains is spectacular: immense peaks of purple and magenta, slashed by deep valleys of vivid green. We drive through an isolated village in a high pass; it seems entirely populated by hordes of children who run in ragged clothing after the trucks, begging. The students, faced with real poverty, have to decide whether to encourage the children or ignore them. They toss out sweets until they see the younger children being trampled in the rush and then in embarrassment try to ignore them. One child runs after the trucks for almost a mile.

Arrival in the capital Rabat brings the first sign of trouble. The aftermath of guerrilla war in the south of the country means that visas to get into Mauritania can only be obtained in Morocco but at the Mauritanian embassy they refuse to grant us visas unless we hold return air-tickets. The cost of 32 air-tickets even if they’d been cancelled after the visas had been stamped, is out of the question. Five pleading visits to the ambassador fail to budge things and expedition members begin to seethe with frustration over eight wasted days.

It’s a disappointment that after so much planning we’re not going to get all the way across the Sahara. Fortunately, the harsh expanse of southern Morocco has plenty to offer and we set off for Laayoune on the Atlantic coast, 2,000 miles to the south.

The desert isn’t what everyone imagines. A lot of it is mountain and rock so that when we camp for the first time in a red moon-like landscape there is a slight feeling of disappointment: “Not what I expected, thought there’d be more sand”; “Ugly and boring”; “I’m counting the days till we get home”. But others say: “Just look at it, it’s amazing, I’ve never seen anything like it.”

We make our way through the rocky desert from oasis to oasis, but routes that are clearly marked on the map are elusive on the ground. A maze of tracks seems to lead in every direction and even the satellite navigator is little help among these rock-strewn valleys. In the empty desert, we see a truck approaching laden with gravel and as we pass, one of the benches lashed to the side of our truck swings out and demolishes the man’s side mirror. We spend four hours while Kevin the mechanic patiently replaces it with a spare mirror and repairs the damaged door. It’s incredible that we collide with the only vehicle we pass in two weeks.

In the intense heat everyone is prey to food and drink fantasies: “A sausage butty with brown sauce and one of me mum’s cups of tea”; “Four pints of cold milk straight out of the fridge and a cake with chocolate icing”; “Gino Ginelli’s ice cream”. After two days spent trying to get to the next oasis at Foum Zgied and with low water tanks, we camp in a hot arid basin among high rocks. But the desert is full of surprises and beneath a palm tree in the wadi we find a well with clear, cool drinking water. We’re not the first to find the well though; a large horned viper, a highly poisonous snake, has already settled in.

Expedition members don’t all share the same attitudes towards wildlife and fierce arguments break out over the rights and wrongs of collecting creatures such as scorpions for souvenirs. Now the snake fuels the argument. Should they “brick” the snake and kill it or share the water supply? The ecologists win the day and the snake lives to provide endless fascination. This “base camp” also provides the highlight of the expedition when members make forays in small groups into the desert carrying all their supplies for two days’ survival.

Just after this there’s a total reversal of climate when a cold wind from the Atlantic drives the temperature down from 120oF to 72oF in a single day and everyone’s rushing to pull on jeans and track suits. There’s an epidemic of stomach cramps and the good humour that has buoyed everyone up so far is suddenly in short supply. One of the girls has developed an ear infection: “Wash yer neck ya scruffy cow,” she’s told by her friend pointing at a tide mark.

When we reach the Atlantic coast, the longest beach in the world, we find a graveyard of wrecked ships, oil pollution and a dead dolphin. “It’s the first dolphin I’ve ever seen and I don’t want to see a dead one with its guts spewing out on the sand,” comments one girl, queasily. An unsettling sight in a dramatic landscape.

The expedition is now heading for home. Behind the big events there’s a constant patina of irritations such as flies, sweat, sand in the food, having to wash in a pint of water, eating three courses off the same plate, doing washing-up duty at one o’clock in the morning. But none of them, in the words of one lad, destroy the deep impression of “a brilliant experience”.

Asked if they would go again, students’ answers vary: “I’m glad I came. I wouldn’t do it again, but it’s something I can say I did”; “It’s going to be lonely back home after waking up every morning surrounded by 32 people - I’d come again”.

Will there be another expedition? Alasdair Kennedy hopes so. Back in 1970 when he first went to the director of education in Liverpool with plans for a school trip to the Sahara he found his application for funding had been turned down. When he asked why, the director threw him a sceptical look and said: “Because you won’t get past Dover, Mr Kennedy.” Twenty-five years later, they’ve passed Dover 24 times and the chances are they’ll be doing it again.

Sahara Journey is due to be shown on Channel 4 in late June.

Total cost of the expedition was Pounds 15,500. For more information contact Alasdair Kennedy at Shorefields School, Dingle Vale, Liverpool 8 9SJ or the Young Explorers’ Trust, Royal Geographical Society, 1 Kensington Gore, London SW7 2ET. Tel: 0171 581 2057 Andy Walker is cameraman and director of Sahara Journey