A not-for-profit group is urging the state to start grading individual schools much like students are graded.   The Children's Education Alliance of Missouri, based in St. Louis, is promoting a plan to give each school an individual letter grade from A to F. That would be in addition to accreditation ratings given each district.  Kate Casas, state director of the education alliance, says even if a district is given full accreditation, individual schools within the district could be struggling. She says 12 states have adopted A to F ratings for its individual schools.  A spokeswoman for the state's education department told The Southeast Missourian the department would need to study the proposal more closely.