Horrified children parted before her as the blood soaked her shirt and dripped from her elbows. An ordinary school day turned upside down in an instant.
Any of us could have done it. A pale girl complains of being unwell so you sit her in the office next door while you continue with the lesson. And then Kirsten takes a pencil sharpener apart and slashes herself deeply on the arms and chest with the blade... there were voices in her head telling her to do it. All the pain and anguish could drain away easily with the blood.
The family has been unravelling for a while. They are in hiding from the father. He has threatened to kill them all. Refuges, restraining orders, relocation, all take their toll.
Her brother has suddenly become wildly disturbed, hiding under tables in lessons, scribbling dark pictures madly. After his first meeting with the educational psychologist he was barking like a dog. We keep him at school as best we can, for there is nowhere else for him to go. Or for Kirsten.
Home for Kirsten is no refuge. All the things that we want our own children to have are missing. Instead there is fear. To put all this right requires turning the clock back, rebuilding a life that has reached crisis point at 14.
I worry about her coursework and attendance. She worries that life might not be worth it. Her psychiatric assessment says she has "suicidal tendencies".
When we became teachers no one told us that the job was about disintegrating families and untrained social work. We thought it was about passing on knowledge. Far from it. We have to be aware of what it is that is preventing us passing on the knowledge. But we cannot do anything about it.
Now we cannot have her in school. She is too much of a risk. If she makes a really determined effort to fall into the darkness can we stop her? How would a teacher feel? How would her class react?
Dealing with Kirsten draws on your resources as an adult but she needs more than we can give her. Plans will be drawn up. There will be meetings, professional voices.
But in the meantime she may wonder. Should she listen to the professionals or to the voices in her head? And she may decide that the voices have an easier, more seductive solution.
Geoff Brookes is deputy head of Cefn Hengoed comprehensive in Swansea