The Day Harry Fell in Love With a Muggle

What happens when an average everyday muggle gets an invite to Hogwarts, when the new headmaster's owl delivers it to the wrong person. Cassie, the girl who got the invite looks extremely like Ginny Weasly. What happens when she arrives at Hogwarts and Harry thinks shes his girlfriend? What will happen to the school when they find out she is a muggle?