*WARNING SPOILERS* The film as I see it...
Ok so this may be too like the film 'Shrooms' but how I see it is...
The Dad/teacher was so desperate to get his daughter back that he decided that he would stage a brutal murderous rampage. We see it how he wants it to be seen ie faceless ninjas murdering and maiming but in actual fact it is him doing it all. When the stabbing happens we see what he wants - his daughter in his arms, his 'demon' dead on the floor. It was in the classroom that she 'came back' to him and so he had to end his other dark side. Or I could be talking *beep* know, numerous flaws...
"It does not do to dwell on dreams and forget to live, remember that."