The deaths plunged the community in the town of Arnstein into mourning.
The five young men and a woman, all aged 18 or 19, were found on Sunday by the landowner whose son and daughter were among the victims.
Police say a wood-burning stove was in use at the hut, but other possible causes are still being investigated.
"The six adolescents... died from carbon monoxide poisoning," said a police statement.
"The cause of the leakage of the toxic gas remains under investigation."
The owner of the hut said he had gone there after his children failed to return from the party the night before.
A neighbour told German media that his daughter had been celebrating her 18th birthday.
The brick building is some distance outside the town and the area was covered in snow.
The tragedy has caused an outpouring of grief in the town, which is about 30km (18 miles) north of the city of Wuerzburg.
An area has been set aside in the local church for flowers, candles and other tributes to the teenagers. A gathering for friends and relatives was held there on Monday "for grief, remembrance and prayer".
Six teenagers found dead in a garden hut in the German state of Bavaria died from carbon monoxide poisoning, police have said.