She did her assigned reading.

↓ Transcript
Selkie: Towards you I roll, you all-destroying but unconquering MATH! From Hell’s heart I stab at thee!

George: Save it for English class, Sel.

Selkie: Bllleeaarrggh! MAAAAATH!

Session Bell: BRRRRRRNNG

Mina: That's the lunch bell, everyone. Lets line up and head to the cafeteria.

Selkie: Oh boy, oh boy! You ready fors this, George?

George: Oh boy, snails, yeah am I ever.

I figure Selkie would most likely have been assigned a watered-down kid-friendly version of the book. Maybe just two stabbings.