Eva Ibbotson has written books for readers of every age. In this latest novel, she has managed to combine at least three traditions, braiding together with delicacy and skill a school story, a war story and a Ruritanian adventure into a funny, moving and uplifting tale which is sparklingly written and full of excitement.

It is easy to identify with her heroine: motherless Tally is brave, friendly, intelligent, and it's not too fanciful to imagine that she is very like Ibbotson herself when young. The author went to Dartington School and its fictional version - here called Delderton - is pretty nearly idyllic, with teachers who might be unusual but who are never boring. Some, indeed, are inspirational. Clemmy, the beautiful former artists' model, and Matteo especially (hardly any surnames in this free environment) are romantic figures with starring roles in the action, which begins when Tally discovers a travelogue about a European country called Bergania, which is trying to stand up to Hitler.

Not only does she want to meet the young prince, Karil, whom she first sees in a picture half hidden by white plumes on his hat, but she realises that there is something she can do to show solidarity. So she and her friends arrange to enter a youth dance festival, just to get into Bergania and see what they can do to foster friendship, peace and love.

In a parallel narrative, Karil himself is miserable about the death of his father, the stuffiness of the court (hilariously described, even though matters become quite serious later on) and his own lack of freedom. He and Tally meet and become friends at once, and Karil takes Tally to the Dragonfly Pool, which is his refuge and escape.

The Nazis arrive on the scene, and punish the Berganians by wrecking the dance festival. What happens after this leads to the most exciting part of the novel, and without giving away the plot, it contains skullduggery of a most amusing kind; scrapes, adventures, misunderstandings, love and friendship and a show in which the myth of Persephone is performed by the children at the school.

The novel is full of delights, with characters who are both believable and heroic. Even Pom-Pom, the dog, who plays such an important part in the denouement, is a fully-rounded character, while the aunts are of Wodehouse standard. The language is neither too formal nor too chatty but perfectly poised between the two, and the moral - that friendship can overcome almost every difficulty - is a good one.

The fact that the Berganian parts of the story are pure invention presents no problems. The sections based on real places give a strong underpinning of historical fact to the fanciful Ruritanian parts of the tale. This "pretend" version of European history is truthful in the ways that matter: it lays out simply and entertainingly the consequences of dictatorship and loss of freedom.

· Adèle Geras's latest novel for adults is A Hidden Life (Orion).