They're only ever locked at night. I guess that, as the school was evacuated, a teacher or maybe the caretaker thought it might be a good idea to contain the potential threat posed by the gunman while everyone escaped.
Even as these thoughts race through my mind, which only takes about two seconds, I am aware that I don't have time to stand and think.
Because if I'm going to make it into the annexe before the armed police arrive, somehow I have to get these doors open.
I wrench the plastic bag of tools from my waist. My first crazy thought is to use the hammer to smash my way through the wooden panels at the bottom, but I realize instantly that this would make a loud noise. So instead I grab the largest chisel and begin trying to jemmy one of the doors open.
I am the most inept burglar in the whole world though, because however hard I bear down on the chisel, even with all my weight behind it, nothing happens. The door does not move a single millimetre.
I am too angry and too fired up even to burst into tears, which is my usual coping technique in a crisis. I realize that there is only one way I'm going to get into the annexe, and it is not through these doors.
Instead I must go outside and find another way in, out of view of the police.
And then I hear it. The sound of a vehicle arriving at the school gates. I don't bother to draw the blind aside a little to look because I already know what it is.
The armed police are arriving.
Somehow I have to get out of the glass corridor and into the annexe before I am spotted, apprehended or shot.
I have only two choices.
One is to return the way I've just come, into the main school building, and then out through the exit near the DT room. From there I can run along the back of the school, past this glass corridor where I am now, and into the annexe through the back entrance.
'Will I have time?' I mutter to myself, still levering at the doors with the chisel, still making little or no impact on them.
Or will the armed police swarm silently around the building and catch me?
There is another way out of here. I am not at all athletic, but I think even I can manage this.
Slipping the chisel back into my bag, I turn to my left, to the large plate-glass windows opposite the ones that have the blinds drawn. The white-painted window ledges are broad and low, almost like window seats, and I jump up onto the nearest one.
Then I reach up to the long, rectangular window above the bigger bottom sheet of glass that does not open. I unhook the latch.
The window swings open and fresh, frosty air hits my face, sending a stream of invigorating energy through me. I grasp the bottom of the frame and heave myself up, my trainers scrabbling for a foothold on the smooth expanse of glass below.
I pause for breath and then manage to swing one leg up and out of the window. I sit astride the frame painfully for a second or two, and then I swing my other leg out. Slowly I let myself down to the ground on the other side. There's no broad window seat outside, only a narrow ledge, so I have to drop further than I would like.
I land on my knees on the gravelled flowerbed under the window, scraping holes in my thick black tights and cutting my hands. But I am out.
Yes!
I can't shout the word aloud, but I punch the air in triumph.
I turn to my right, back towards the annexe. Keeping close to the building, I edge my way along the length of glass corridor to the side wall of the annexe which juts out beyond it. I press myself nervously into the shadow of the building and take a couple of breaths.
Then, glancing nervously from side to side like a frightened bird, I slip sideways and round the corner of the annexe.
There I stop and look up. I am almost directly below Class 9D's form room on the first-floor corner of the annexe. I know the classroom well. I had French lessons there when I was in Year Eight.
But I can see nothing because the blinds are drawn. In fact, I note that the blinds are drawn