Sounds like she is taking a lend. You have had some good advice already - maybe think about terminating the lessons, she isn't worth the hassle.
I had one last week who phoned 15 mins before the lesson (she lives 5 mins away) and said, sorry she couldn't leave her children because they were feverish and the husband had been called away at short notice so couldn't stay with them. This week she turned up with this week's money and last week's - no quibbling, nothing. Just paid up and then got on with her lesson.
And the other cancellation from last week (actually a no-show who phoned a couple of days later) came today with 2 weeks money and then explained that she had just found out she has cancer and had to have chemo on the day of her piano lesson and the piano lesson had slipped her mind. She is going in for an operation in 3 weeks and wants to continue with the piano to keep her mind off it but can't guarantee to come each week - so we agreed that she would keep her slot (first thing on a morning) and either come or not come depending on what treatment she is having and how she feels and I won't expect payment for missed lessons while this is all going on. She is a great adult student and I want to help her in anyway I can.
But that is a whole different kettle of fish to people who mess about - I have had plenty of those and I have had to toughen up and I have parted company with those who thought they could just randomly cancel and not pay while I was sitting at home waiting for them and worrying about whether I would have enough money for the bills and food!