Emergency Alarm for Number 28

There's just PE to get through, and after that it's the Easter holidays, at long last! But then a supermarket carrier bag containing the whole class's mobile phones disappears - the sports teacher had collected them for safekeeping. Now all the phones are gone. Before long, suspicion falls on Joran, the new boy in class - for one thing, he doesn't seem to have a mobile of his own, and for another he has mysteriously vanished as well. It's clearly a case for Nadeshda and her friends. The young detectives soon realise that Joran is innocent. But who is the real culprit?