The fire brigade is treating it as suspicious. I appeal in assembly for information and, unusually, as our intelligence network is normally ultra-efficient, I get no clues. I suspect no students are involved, but curse those who are.
Tuesday: The phone rings at 2am. I am told that "the music room's on fire". The shortest conversation on record with Jim ends with my automatic response, "I'll be right there".
Twenty minutes later, I arrive, unwashed and unkempt. The fire is still burning; the music room and headteacher are gutted, and smoke is pouring through the rest of the school.
At 2.30am I tell the local education authority. People start arriving with help and support. At 4.30am, it's the director of education. She is dressed ready for a day's work and proffers excellent advice.
At 6am, we contact staff. I leave a message on the answerphone for the music teacher. How do you tell a young, enthusiastic teacher that his teaching area and all his instruments are ruined?
From 7am, staff arrive and are dispatched to various parts of the school to determine the level of damage. I tell them to prepare their insurance claims and not to worry about the amount of ink they use.
Later, health and safety suggests staff should leave, which most do, as long as they are "at the end of a telephone". The expressive arts teachers remain to thumb through inventories and catalogues. A security patrol is provided for the site.
Wednesday: Expressive arts staff continue their efforts. Others clear the smoke-damaged staffroom. I walk around and the extent of the damage becomes clear. It's not just the music room - if it were, things would be manageable.
Water damage to the school hall and smoke damage to about a third of the rest of the school, including the library, raises the question, "When will we get the students back?" Much is destroyed; the music teacher's violin, given to him by his grandparents, a victim. That will upset the kids.
The level of support from the community, parents, students and fellow heads overwhelms me. Numerous contractors are now on site. Jim will monitor them.
Thursday: The 24-hour-a-day cleaning has started. Support keeps streaming in. Ironically, a meeting with the parent-teacher association decides that the planned firework evening will continue. Jim is concerned about the progress of the repairs.
At 8pm, the police tell me they have found our school's security markings on a keyboard found in a 16-year-old's house. I am asked to identify it. I defer to the music teacher, who visits the police station and provides sufficient evidence for an arrest.
Friday: The first of three mobiles has been erected. The expressive arts team finishes the ordering of new equipment. A youth is charged and is bailed until the end of November. The security patrol is removed. No decision is yet made about the return of students to school.Jim continues his relentlesspursuit of contractors.
Ashley Pellegrini is headteacher of Ashdown School, Poole, Dorset