Tania Gill had been dropped off at the Convent of Jesus and Mary school in Sialkot, Punjab, by her brother, Johnson, at 8am on the morning of her death.
He says he was then called by police in the afternoon and told that his sister had died and he had to identify the body.
Officers said Tania's death was suicide and she had jumped into the canal.
Speaking through the British Pakistani Christian Association the family say the police "blundered" the investigation and they do not believe the girl killed herself.
According to BPCA her relatives have "several factors" which leads them to believe she was drugged, raped and murdered.
Family members say her dead body showed the after effects of a date rape drug and that her trousers had been ripped.
They also state that police claims Tania was in a "deep depression" are false.
Her father Nadeem Gill said: "Tania was young, happy and full of energy. There is absolutely no way she would take her own life.
"Our corrupt police force is preventing justice for my daughter. But I will never give up till her murderer is found."
