These are characters who will appear in the book for cameos (and possibly more!) in no particular order:

Timothee Chalmet as Tomas Rosewood TomasGranger003 (Slytherin House)

Madison Beer as Mia Blackwood HermieCrazies  (Slytherin House)

Curran Walters as Axel Leroy-Smith PeaceByFreak20  (Ravenclaw House)

Adelaide Kane as Selena Raywood -we-are-infinite-  (Gryffindor House)

Karan Brar as Samuel Auras  ElectricZenith  (Ravenclaw House)

Nam Joo Hyuk as Kieran Winston kieran_w  (Gryffindor House)

Martha Higareda as Ashley Zermeno CHXRRYASH- (Hufflepuff House)

Brendon Urie as Cody Greyback Stargaryen1  (Gryffindor House)

Inigo Pascual as as Reimond Gonzaga SEKRETONGMALUPYET  (Gryffindor House)

Kieran Shipka as Thea Stark-Hart mickey_007i  (Ravenclaw House)

Kim Tae-hyung as Elijah Farrell "Just Farrell"  Aeolusss  (Ravenclaw House)

Cameron Monaghan as Aidan Korff JasonTheRedHood  (Gryffindor House)

Alex Hook as Kayla Rosario Kyle182099 (Ravenclaw House)

Tom Welling as Charlie Kynaston BeornRedcliffe Gryffindor House)

Louis Partridge as Matthew Stone Emioneforever  (Hufflepuff House)

Rahart Adams as Luke Phoenix Lukeybboi (Gryffindor House)

Matt Lanter as Leon Grimwood Shototodoroki00  (Hufflepuff House)

Caleb McLaughlin as Koen Moore DIOJoestar420 (Slytherin House)

Chloe Grace Moretz as Andy Lingstone wonhosmila  (Gryffindor House)

Naomi Scott as Leo Marcel HedaSbish (Slytherin House)