So my thinking is Sarina is a tsundere. When Aya first joined their class I remember she smiled at her and looked like she wanted to be her friend. But Aya being the awkward girl that she was ignored her and unknowingly offended her.
Though, not a great justification for her bullying especially the shit they'd do to her and even attempting to get her raped. That's some really messed up level of tsun, before the dere