Polly Chapman boards the Hogwarts Express on Platform Nine and Three-quarters from King’s Cross for her first year at Hogwarts School. She is initially pleased that they have a Potter in their year – until Albus is sorted into Slytherin house (CC1.4).
It is not stated specifically whether Polly is sorted into Gryffindor or Slytherin House. However, she seems to be a Gryffindor because later she is upset that they are losing points because Albus “hates Gryffindor” (CC2.11).
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 31 July 2016, with Claudia Grant in the role of Polly Chapman: Harry Potter and the Cursed Child, Parts One and Two