Beware of the Demon Headmaster(2002)
(A book in the Demon Headmaster series)
A novel by Gillian Cross
Sally and her twin, Ben, have been looking forward to their school trip to Notley Castle for ages. When the day arrives though, Ben is ill, so Sally promises to text message every hour to let him know what's happening. When her messages start sounding strange, Ben decides to go to the rescue.
Used availability for Gillian Cross's Beware of the Demon Headmaster
June 2002 : UK Paperback