Police said it is possible the 16-year-old, named locally as Jodie Muir, may have taken an "ecstasy-type" drug at the gathering in Rutherglen, South Lanarkshire, on Friday night.
She returned home at about 5am and died several hours later. Tests are being carried out to establish the cause of death.
Her mother Laura McArthur has hit back at suggestions the incident was drug-related, saying that her daughter was suffering from a heart condition, Long QT syndrome, which can cause abnormally rapid heart rhythms, sometimes resulting in death. Gordon Shaw, the headteacher of Jodie's school Eastbank Academy in Shettleston, Glasgow, said: "This is tragic news and our thoughts and prayers are with Jodie's family and friends at this very sad time.Jodie was a popular and valued pupil, and we will be offering any support that is needed to her classmates and teachers."
Police Scotland spoke to a "significant" number of the 40 to 50 teenagers believed to have been at the party.
Chief Superintendent Ciorstan Shearer said: "I do not yet know whether or not drugs have played a part in this tragic death and it is therefore important to trace other party-goers to establish that they are safe and well. At this time we know that there were around 40 to 50 other teenagers at this party in Rutherglen and we have managed to speak to a significant number of them.
"It is vital, however, that those who have not already spoken to police come forward as soon as possible, first and foremost to ensure that they are safe and well, but also because they may be in a position to provide vital information to assist the ongoing investigation."