Boredom wasn't a terrible explanation for why Serafina was interested in politics. The simple answer was that she'd set her political dreams aside in favor of her son, though it was somewhat more complicated than that. She had certainly been involved in the school while Blaise was there, but not formally; keeping up acquaintances with teachers and members of the board of governors had been about as far as she'd gone. At the time, she'd reasoned that she couldn't get so occupied in the politics to overlook her son's best interests (which she thought she was likely to do, since the political world called to her like a glass of fine whiskey to an alcoholic, even though she'd never gotten a chance to be addicted).
It was only recently that she'd truly felt Blaise was not only independent, but wanted much less of her influence on his life - and if there was one thing Serafina refused to be, it was a parent like her own had been. She wanted Blaise happy, at any and all costs; if the Zabini name had to take a blow in order to make that happen, she would do her best to make it sting as little as possible, but she wouldn't hold it against her son.
With him making his own way in the world, however, Serafina had taken one look at her life and decided not to fall into the same patterns she'd been following for years. Her name was good, and she had no worries about finances; probably most importantly, she was neither married nor being pursued (and she certainly wasn't pursuing anyone, herself). She had recently turned 41 (though of course, she never gave her age or made a big deal of her birthday), and as her present to herself, she was going to give her childhood dreams a real chance of coming true.
She arrived outside of the gates at Hogwarts with very little ceremony, and was escorted inside to the Headmaster's office. Her smile when she knocked was polite, but it was also genuine; although they'd lost touch over the years, Severus was one of the handful of people she still respected. If her political career would mean that she saw him more often, she certainly wasn't going to complain.