How is this for my prolouge?

It's good. Most people are able to relate to the beginning and you later draw in element of mystery. My criticism is mostly with structure. It seemed a bit choppy to me, the sentences need to flow together a bit more. I hate to say it because I will sound like my English teacher however a transition word may help there. Besides that the first sentence sounds a bit off. You could try 'Grace sat in the large clroom, absentmindedly tapping her fingers...' because with sat in the past tense you have already set the action in the past and so tapping can be present. If you don't like that you can try 'Grace sat in the large clroom and absentmindedly tapped her fingers..." the "and" makes it flow better. Try reading it aloud or leave it for a bit and come back to it. Often you cannot catch your own mistakes after writing because your brain is attuned to what you wrote and sees it as correct or something. (English teacher once again). Hope that helped a bit :)