sounds like a decent idea. there's a definite objective of the protagonists with a sense of urgency considering it's a disease, so the story will be gripping hopefully.
What sort of magic/force is Ari? (I'm reminded of the Japanese phrase "Nandemo ari", meaning "Anything goes", but in this case, it means "Everything's magic" XD)
I think it's great that the main character's can't use magic, because it makes their challenges more interesting than just a power fight all the time.
Also, what era is this? you mention Knights which implies medieval, but also police and schools which imply modern-ish.
It sounds like you have a lot going on in your first chapter, you might find it hard to fit it all in unless your storytelling pace is really fast, which might make it hard to follow. Each of those four paragraphs sounds like it could be at least a chapter in it's own right.
they end up doing something to her in order to keep her quiet.
Do you actually know what they do yet, like a threat or magic or something?