Such extraordinary juxtapositions of the worlds of adulthood and adolescence have become everyday life for the Manchester trio of Zainam, 17, Cleo, 16, and Yonah, 14. Never mind that they have had three top five singles this year and been special guests of the Spice Girls on tour - they still have their schooling to complete.

And if the girls should forget, their mother and constant chaperone, Christine Higgins, 36, and their full-time on-the-road tutor, Barry Withers, are on hand to remind them.

Precocious beyond their years, the girls have a slick stage performance that can easily obscure the fact that under normal circumstances they would be more concerned with pocket money and getting a Saturday job in McDonalds than signing a million dollar contract with Madonna’s American record label and promoting their new album, Comin’ Atcha!

Michael Jackson has said that he feels he was “robbed” of his childhood. Backstage before their show at Sheffield’s Don Valley Stadium, I sit in on their English lesson (enlivened by a series of comic voices as they grapple with Shakespeare’s text) and ask if they feel similarly deprived.

“No!” they yell in unison - but then go on to detail all the teenage things they miss. They cannot go shopping because they are mobbed by young fans. They have no one to play netball with. They don’t have time for boyfriends. And their childhood friends treat them differently whenever they return to their old Manchester haunts.

“They jump on our tour bus and shout ‘Look it’s Cleopatra!’ We say, ‘Oh come on, please, we used to go to school with you’,” says Cleo.

The girls were taken out of their Manchester comprehensives in summer 1997 by agreement with the local education authority after they signed a recording deal with WEA. They now have two private tutors, one for when they are at home and Withers, who travels with them whenever they are on tour. These days that is most of the time; in the past eight months they have made two working trips to America and two promotional tours of Europe,as well as sell-out dates around Britain.

Yonah, at 14 the youngest, has two years of school left and is legally required to have 15 hours of lessons a week. Before they tour abroad, representatives of their record company must attend a magistrate’s court and give formal undertakings that the curriculum will not be neglected. Although Zainam and Cleo are now of school-leaving age, both have decided to continue their education.

I ask whether they miss school and my question is met with derision. “How can you ask a teenage girl that?” Yonah demands. Yet it is clear that they do miss it. “We used to watch MTV with our friends in the hall during break,” she says.

Cleo chips in to say that although their private tuition is good, there are no classes in drama, art or home economics. “It’s important that we do school, ” says Zainam. “I didn’t do very well in my last exams so I decided to re-sit them. We’ve had a laugh reading Macbeth, and Cleo is a maths freak.”

Her indignant younger sister denies it and then turns defensive: “If our career disappeared tomorrow, we could go and become secretaries in an office or something.”

Withers, a former college lecturer who has been teaching the girls since last November, says they are attentive pupils, despite the excitements of their pop star existence. “What surprises me is that they seem to have the ability to switch off everything else and concentrate on their lessons as a self-contained activity,” he says.

The plan is for the older girls to take GCSE English and mathematics next June. “We share the work between two tutors, but since the beginning of the year they have spent so much time on tour that I have really taken responsibility for the curriculum,” he says.

Lessons are frequently disrupted. “The tuition gets done, but not always in the best way or in an orderly fashion,” Withers admits. “It is built into the schedule and is meant to happen first thing in the morning when they are alert. But if there are press interviews or photo-shoots all day, it can be the evening before they get down to their books.”

Lessons can take place on the tour bus, in backstage dressing rooms or in a hotel room in Rome or Los Angeles. “That might sound exotic, but it is exactly the same curriculum they would be doing in a classroom in Manchester, and I’m doing the same job,” Withers says. “They know the importance of what they are doing and they never complain about it.”

Christine Higgins confesses to some worries about her daughters’ lifestyle. “Rock ‘n’ roll often goes with drink and drugs, but they are quite balanced and they haven’t got any airs and graces, ” she says.

“If they remember what they have been taught and keep their feet on the ground, they will be all right.”