Gabrielle Price, 14, might have taken a mixture of ketamine and a new party drug 'meow meow' before she fell ill, according to neighbours.
The 'legal' drug, also known as Mephedrone and MCAT, can be bought over the internet and is billed as a cross between Ecstasy and cocaine.
Gabrielle, known as Gabi and described by friends as 'fun-loving and hyper', had been out with friends all day before attending the party in Brighton, East Sussex.
The brunette, from nearby Worthing, West Sussex, suffered a fatal cardiac arrest after falling ill at around 5.15pm on Saturday.
Ambulances crews were called and battled to save the teenager at the house before transporting her to nearby Sussex County Hospital, but she passed away at 10pm.
A Neighbour, who did not wish to be named, claims Gabrielle had taken ketamine mixed with the new party drug 'meow meow' before she fell ill.
A 39-year-old woman and a 17-year-old man were arrested at the scene on suspicion of drugs offences while pathologist carry out a post-mortem to see if Gabrielle had drugs in her system.
Sussex police Detective Inspector Bill Warner said: 'We have suspicions there were two people supplying drugs but not necessarily to her.
'The post-mortem test will see if her death was drug-related or natural causes.'
Next-door neighbour Anita Cullen witnessed paramedics' frantic attempts to revive Gabrielle.
She said: 'I saw a whole load of blue flashing lights outside. There were four or five paramedics and a couple of ambulances.
'Then they brought someone out of the house on a stretcher
'She was all covered up; you could just see her feet. She didn't look in a good way.'
Della West, headteacher of Davison High School, Brighton, where Gabi was a pupil said: 'She was a popular girl, very quirky, with her own individual sense of style.
'She loved music and was hoping to use her creativity in a future career. She will be sadly missed.'
Former school friend Amber Herriott, 16, said: 'At school she was always really happy and hyper.
'She was the most beautiful angel I knew. I hope she rests in peace.'
Class B drugs include amphetamines and cannabis which was re-classified earlier this year.
Possession of the Class B drugs carries a maximum five year prison term, an unlimited fine or both, while dealing the drugs carries a maximum 14 year jail sentence.
