She is enrolled in a public school receiving a free education. A requirment of THAT enrolement is to be there. She repeatedly skipped classes, went before a judge once before and was under a court order to not have anymore unexcused absenses. She failed to comply with a lawfull court order and since she is a legal adult she recieved a penalty that is consistent with being an adult. As an adult she has the choice of either following the court order to be in class, or dropping out of school. She choose to stay in school, so she was expected to follow the rules. A weekend in jail sounds like an appropriate punishment and she will either learn that her actions have consequences, or she'll end up right back infront of that judge and spending more weekends in jail. The choice is 100% hers, as is the outcome.
What's her mother going to say when she gets a job and repeatedly skips work and gets fired? The punishment is unfair because when she is at work she does a good job? Funny but I doubt that will fly very well.