Inside the Worm by Robert Swindells.
Inside the Worm (1993) by Robert Swindells is a thriller with a supernatural twist, set in the English town of Elsworth.
Fliss and her friends are performing in a school play about the legendary Elsworth Worm — a dragon-like creature said to have terrorised the village long ago. Four children are chosen to operate a large worm costume in the performance. But the costume contains something ancient and evil — a real worm-spirit that awakens during rehearsals and begins to take control of the children inside it. The children start behaving strangely, becoming cold and distant, speaking with one voice. Fliss, who is not inside the worm, must find a way to save her friends before the worm completes its possession of them.
The legend of the Elsworth Worm is based on real historical events. Fliss researches the legend and discovers how the worm was originally defeated — and uses this knowledge to save her friends during the performance.