It's the usual chaos backstage. Costumes and props spill out of boxes; the level of talk is high. In the wings, some of the actors study their lines, check their cues. Others are onstage, rehearsing. The director calmly takes them through it again, from the top, one more time: "And a star shone over the stable . . ."

The children of Killaghtee Church of Ireland school, near Killybegs, in Co Donegal, are preparing themselves for the annual nativity play. There is only a handful of children in the school: nine boys and 17 girls, aged four to 12. Everyone has a part. Some even have two.

Grβinne, her head coming up to Joseph's elbow, is seven and plays Mary. "She wanted the part so much," says the head teacher, Nuala Dudley, "so I gave it to her." Grβinne is bright-eyed, determined, not one to beat about the bush when it comes to recognising the importance of the star part. "I wanted to be Mary because she's very important," she says.

And she has a compelling stage presence, looking devout when called on to do so, maternal on other occasions, accepting the gifts of the Magi with regal graciousness, reverting to wrapping and rewrapping her doll, otherwise known as Baby Jesus, when there's nothing else to do.

Grβinne is generous about her co-star, Martin Shaw, and prepared to share the limelight with him. "He has to be there, because everything's happening and things are going very fast for Mary and the baby, and Joseph doesn't just ride on the donkey, he lets Mary ride on it as well. He helps her, and he has to find somewhere for them all to stay the night."

Standing beside Grβinne are the three wise men. Or, rather, two wise men and one wise woman. Abby is the wise woman, and when I check her name she gives me the whole thing. "Abby Sarah Rachelle Deane," she says, and I think I'd better check with Grβinne in case I've left out any of her names, but she puts me right. "I'm Grβinne Nelson. I decided when I was born I didn't want a middle name." So that's OK.

The children are old hands. Everyone has played some part or other in the past, and they're used to doubling up as well. Dudley has written the script and adapts it every year, according to the number of children in the school. Shepherds, angels and locals can swell the crowd scenes when necessary, and if there's still someone without a part they can be an usher.

She takes the two narrators through their lines, and in the "scuffle corner" three of the boys have a sort of silent wrestling match, with their feet, while sitting quietly, as they've been told. Then Mary and Joseph go into a huddle with their teacher, and one spark has the brilliant idea of hiding Joseph's lantern in the manger, underneath Baby Jesus. He thinks of hiding Baby Jesus as well, but cops on that that would be a trick too far. Mary is not amused and goes into maternal overdrive, furiously rocking the cradle to restore dignity to what, after all, is a serious play.

Killaghtee is a two-class school with one junior infant and two senior infants, the rest going onwards and upwards to three 12-year-olds in Class Six. The junior infant is four-year-old Zara Mary Margaret Henry, who plays an angel. She thinks hard about what an angel is, stretches her arms behind her head to help her think better, blows her hair out of her eyes, thinks a bit more and then comes up with the only logical answer: "It's a girl." Zara will wear a white dress and "a tinselly thing" on her head and already knows her momentous line: "Peace be unto you." She has to say it when Jono the shepherd finishes his bit.

There are quite a few shepherds, and they all have staffs. Nobody is sure what a staff is for, but they're handy to twiddle and spin if you're nervous or when things get a bit boring. Not that there's much time to be bored. There are crowns to be made and Christmas cards to be coloured. The big ones have the job of looking after the little ones, and Kerri Maxwell is stationed in the information-technology corner - "r∅omhaire", says the card on top of the computer - checking the printer to make sure the programmes are coming through with everyone's names spelled correctly. Kerri also plays the part of a local. "That's someone who's there so it's not just Joseph and Mary," she explains.

Dudley rings the handbell on her desk to get a bit of hush and the older children sing a jolly Christmas song; the more serious of the younger ones, frowning in concentration, mouth the words, fingers moving along the line, from word to word. Standing beside the organ, the sole instrumentalist, 10-year-old Edel Rodger, taps away dreamily on the triangle with the music. He doesn't count the beats. It's more a rhythm thing. And he shows me how you swing your hand out in an arc and somehow connect with the triangle at just the right moment.

Mark Shaw is Zacarias. He's not sure who Zacarias is, and for the life of me I can't remember who he is either, but Mark says Mrs Dudley knows and, luckily, she does. Zacarias is married to Elizabeth, Mary's cousin. Mark has to say, "But Elizabeth and I are too old to have children", at which point he is struck dumb, probably because the Angel Gabriel has just told him the startling news that he is about to become a father - of John the Baptist.

In the midst of the singing, organ-playing and triangle-tinkling, and of people saying their lines and playing about with poor Baby Jesus, tossing him through the air in an attempt to annoy Mary, 12-year-old Adam Deane grabs a minute to himself and goes into a corner to catch up with his Harry Potter. Adam plays the baddie.

A baddie, in a nativity play? "Yes, I'm Ceasar Augustus, the Roman Empire." Beside him, Eva Deane checks her notes: "Funky BadGirl", it says in the margin. She has just turned 12 and is not sure where she'll go next year. She could go to Killybegs, Donegal or even boarding school in Raphoe. Whichever, she's not worried. Nobody in Killaghtee school seems worried about anything, least of all forgetting lines on the big night. "We just say them over and over again and then we know them," someone said with the insouciance of youth.

"They have no inhibitions," says the Reverend Ken McLaughlin, who has dropped in to see Dudley. "It's only us adults that have inhibitions."

Us adults look glumly at each other while, behind, a bit of noise breaks out in the scuffle corner. "You've probably noticed the boys are a bit tired," says Dudley, displaying the patience that saintly teachers are born with.