If it's Sabrina she could start to get more and more obsessive over her control. As Britney grows more dependent and starts believing anything Sabrina tells her even altering her world views to suit Sabrina's wishes, Sabrina grows intoxicated on her power. It starts with just making Britney do humiliating things and the rush that comes with Britney caving in and doing them. Then the feeling of Britney no longer fighting her in these requests, she is becoming obedient without question. Sabrina finds she can force Britney to say and do anything, Sabrina's word is law and Britney will conform to her designs.
But it's not enough, during this time Sabrina's public social life is taking off as the new cheerleading captain and school's new queen bee she's enjoying a similar power trip in school. But her control over her fellow students isn't as strong as what she holds over Britney. The more she wraps Britney around her finger the more she craves that level of power with everyone. It's especially hard when some students will outright defy her or even ignore her perfection as if they have lives beyond hers.
She wants to have more Britneys, she knows she can make Britney do anything, even help her "fix" those students that don't know their place. She could do it, just get them to come to her house and with a simple command Britney would gladly help make them understand their place just as Britney does. She could have a whole pig pen of obedient pets that know their place.
If only there were someone at school with a type of power that could make the whole school her pig pen...