The issue here is not her faith. The issue here is whether she
can do her job.
If she doesn't want to listen to the child read Harry Potter, she
can resign. However, she wants to keep her job.
The US has what is called the Establishment clause in the Bill of
Rights. It states that "Congress shall make no law respecting an
establishment of religion". As a teacher in a public school, she
cannot allow her religious views to influence how she does her
job.
Another example is school prayer. Students are allowed to pray in
school. However, the school cannot announce through the PA system
"let's all bow our heads in prayer." Teachers also cannot lead
prayer.
If you break this separation of church and state, it opens a whole
can of worms. For example, a teacher might be able to reject a
reading of "The Canterbury Tales" because it draws many allusions
to the Christian God and the Bible and the teacher believes in some
other god.
