Scout is discussing with her father the annual farce of Burris Ewell's turning up to school on the first day of term to get his name on the list of students and then effectively being allowed to play hooky for the rest of the term by the education authorities. Scout doesn't think it's fair; why shouldn't she get a similar break?
Scout tells Atticus that the truant lady reckons that she's carried out the law by getting Burris's name on the school roll. But Atticus knows different. He knows that the truant lady is actually bending the law, which in his view is necessary in certain cases.
It may seem strange to hear a lawyer say such a thing, especially one with as much integrity as Atticus Finch. But Atticus is also a very wise man, and he knows that in the real world, it's often necessary to bend the law a little in order to make things function properly.
In practical terms, it would be impossible to apply the law strictly in Burris Ewell's case. A member of the notorious Ewell clan, the disgrace of Maycomb, Burris shows no signs of wanting an education, and so, according to Atticus, it would be silly to force people like the Ewells into a new environment.
Although Atticus greatly respects the law, he also knows that it's far from perfect and does need to be bent in certain cases. At times, the law can also be unjust, especially in relation to the treatment of African Americans. As we will see during the trial of Tom Robinson, the law in the Deep South at this time was used as an instrument of white supremacy and as a way of keeping Black people in line.
Having Atticus undermine—albeit ever so gently—strict adherence to the law allows Harper Lee to hint at the possibility of not just bending the law, but also breaking it where it is manifestly unjust. As a supporter of the civil rights movement, we would expect nothing less from her.