The reasons for a school to ban a book from their libraries or classrooms can be a lot, like, unacceptable languages, moral issues, negative activities, profanity. When the schools believe that the book will make bad influence to the students who read it then they will censure it. The schools do this to protect their students from bad or improper conceptions. And also avoid them to be in trouble with parents’ anger to students’ complaining.
It’s hard to say if a school to remove a book for its own reason is appropriate to not. On one hand, the school removes a book for the reasons it believes are good to students. The school does the thing which it thinks is right to ban those violence and sexual graphics and plots. So it can keep the students innocent and pure. On the other hand, by doing this the school cut the students’ right to know what they can or want to know. And they will try to get the book outside school and without a teacher’s teaching, the situation may be worse. Also by ban a book, the school may stop the students to read some really good and meaningful books.
What to teach and what to read are depending on how the book being taught and who are going to read it. And the situation and boundary in different times can be different. Since in different eras, people have disparate ethical and moral value. Those are also very important factors to decide reading a book is proper or not.