Assigning students in a public school to copy the shahada is a bad idea. But closing the school over it is ridiculous (as is this level of parental outrage). Get the teacher to revise the assignment and move on would be an appropriate response.
On the other hand, it is pretty hard to find an example of Arabic calligraphy that isn't Quranic, or at least religious. I have a t-shirt in Arabic calligraphy and part of the joke is that it is in a classic calligraphy style but has nothing to do with religion.