Karl Jenkins boards the Hogwarts Express on Platform Nine and Three-quarters at King’s Cross and starts his first year at Hogwarts (CC1.4).
It is not stated whether Karl is sorted into Gryffindor or Slytherin House. However, later on he does call Albus Potter a “Slytherin Squib”, which could imply that he wasn’t pro-Slytherin (CC1.4).
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Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 30 July 2016, with Jack North in the role of Karl Jenkins: Harry Potter and the Cursed Child, Parts One and Two