There are only two girls and teachers left.

The living female students said that the teacher was not good to these students before.

So these students came back for revenge.

They were all killed by the deformed children born by the teacher. But later

The girl student who was found alive by the police killed everyone.

Because the teacher was not good to her because of her poor family.

And indirectly caused her mother's car accident.

So she killed the teacher's favorite student (she bullied her on the teacher's side before)