Tania Mariyam's family had consistently said the girl was drugged, raped and murdered but police had refused to investigate.
Despite evidence of her clothes being ripped and signs of drugging, officers maintained she had killed herself.
She was found in a canal hours after being dropped off at the Convent of Jesus and Mary school in Sialkot, Punjab, by her brother.
The family has been battling ever since to raise the profile of her case and force police to investigate.
The British Pakistani Christian Association, which has been supporting the family, fears the delay in reclassifying the case could have let the killer escape.
"I am worried that the severe delays will mean that much of the evidence has been lost, especially with regards to the toxins in the decaying corpse of Tania," said the organisation's Wilson Chowdhry.
The girl's father, Nadeem Gill, said: "There was a disgusting police cover up and I fear that they have colluded with the murderer and know more than they are letting on. They do not care about Christians."
