The dance the Abbey Mount girls perform before the last part of the lacrosse Finale is a modified Haka, or "Maori war cry", as used by the New Zealand All Blacks rugby team. The first New Zealand rugby team to play overseas back in the late 19th century was the first to perform the Haka before their matches against the Australian teams to intimidate them. The Girls' Haka contains several of the present day's All Blacks' more aggressive Haka; Standing with knees bend while holding their arms bend over each other and their hands fisted, sticking out their tongue while yelling at the other team, slapping their elbows, thighs and chests, kneeling on one knee with a fist in the ground and their threatening facial expressions.
Actress Daisy Donovan, who also appeared in the film, contributed some additional dialogue to the screenplay, mainly consisting of contemporary phrases or sayings. She said one of the ways she did this was to go to the library and listen to teenagers talking to their friends whilst revising their A levels.
Harriet dresses as Keira Knightley from in her role as Elizabeth Bennet from Joe Wright's Pride & Prejudice (2005). The music that Mr. Nellist plays is "Dawn" written by Dario Marianelli which features in the film. Pictures from the film are seen as Harriet designs her costume in her bedroom before the "coffee aging" is poured on Poppy (Emma Roberts).
Early in the film, Poppy makes a reference to the school being just like Hogwarts (from the Harry Potter series), just moments before Matron played by Shirley Henderson appears on screen. Henderson played Moaning Myrtle in Harry Potter and the Chamber of Secrets (2002) and related films.
There is multiple references to Alice in wonderland throughout the entire film. The main reference is more of a hidden one. Alice in wonderland had one family that we knew the full names of was the Kingsleighs. Which Freddy and Mrs. Kingsley have the same last name too, the only difference is the small difference in spelling.
Poppy's quoting from the end of Pride and Prejudice shows that the film was written by Americans as it's from an alternate ending to the film that was added for US audiences and wasn't in the UK version.