Locker credibility concerns? Go low, not high.
Conscious conspiracies, explicit tradeoffs, deliberate action... these can be fun, these can be fascinating, but they are not the default setting for many.
Most people make startlingly few deliberate decisions, letting habit, convenience, and imitation handle most of the 'choices' made; most everyone sees what they wish to see.
So work from there: initial incidents have no evidence (and besides, investigations are inconvenient), and that shifts T from 'victim' to 'annoying girl who keeps making unprovable accusations', this becomes habit, imitated more widely (Teacher A: 'Hey, T said something about bullying'; Teacher B: 'Yeah, she keeps crying wolf about that). Meanwhile you have productive girls, who get their homework in on time, who run track, who smile nicely, who are so very much more believable, who tell a different story. And, after the loss of her mother, is it any wonder that T is acting out? Seeking attention, even negative attention? Doing worse in classes and on tests?
The locker, of course, is harder to dismiss... but at that point, admitting to the ongoing issue involves admitting to the ongoing dereliction. Much easier to say (correctly) that there's no hard evidence, and let the petty passive aggression available to any minor member of a large organization do the rest by sheer virtue of inertia. There's no need to bury an investigation when just going through the motions will achieve the same effect.
All of which is to say that Sophia doesn't need to be a Ward, that Blackwell and Gladly, et al., don't need to have secret motivations for turning a blind eye, for the bullying situation to occur. Indeed, given a T with no faith in authority, one who actively rejects milder disciplinary options (Suspension? That'll just make things worse!), it's not hard for those worthies to shrug and think 'I tried', or 'not my business, really,' following any half-hearted attempt to help T (or, more cynically, assuage the dull prickings of conscience that an effort was made).
Bottom line: setting up a world in which a non-Ward Sophia gets away with things is not an objective question, it is a question of chibipoe's skill.