If everyone in her grade is going - students and teachers - what is she supposed to do if she goes to school? Who is going to teach her or supervise her for the day? That's more than likely why they're having those kids stay home who choose not to go, there's nothing for them to do at school that day and no one to supervise them.
Go to another class. When I was in 4th grade I forgot to get a permission slip signed and spent the day in another classroom.