Banishment

Before Weirdmageddon, Bill and his wife, Decell, had two children named Diana and Basin but when Bill and Decell get banished into another dimension and Bill asks his father, Ci, to take care of his children.

As Ci takes care of Bill's children he raises them as his own like his own children(Bill, Will, Clarissa, Crystal). He sends them to a school called Mortal and Monster School which is a Pre-K-High School where they meet there new friends Dipper and Mabel.