This really has nothing to do with it being the Quaran.
If it was ANY other book BUT the bible, parents would have complained. If it was a bible verse, saying that the Christian God is the only God, they would have been fine with it. In fact, they would have said she was the best teacher ever!!
Hell, if they were told to copy a quote by the Buddha, such as, "Your purpose in life is to find your purpose and give your whole heart and soul to it," Christian parents would have complained because it didn't come from the bible.
Personally, there is probably a different way to teach world religion than to have them copy down a saying. The teacher could have went about it in many different ways. However, we all the know the truth as to why the parents were complaining.