If you allowed the situation to get to the point that a window shade was the last straw, then you allowed the situation to go to far.
Without going into any details, how many times had you sat her down prior to the dreaded curtain incident, and how did those sessions go? Bottom line is that you control the school, not her, and you should have been able to come to an understanding before it became necessary to kick out a student.
Of course, if you sat her down more then a few times and explained things with her and she still chose to challenge your authority in the school, including disrupting class and bothering the other students, then you have no choice but to ask her to leave, and perhaps remove her child from your school.
As to what exactly was the last straw, it doesn't really matter - what matters is that the line had been drawn and she continued to step over it.
As to the rest of your post, it sounds like this is what Terry did.