What is the number of classes offered by each school?
SELECT count ( * )  ,  T3.school_code FROM CLASS AS T1 JOIN course AS T2 ON T1.crs_code   =   T2.crs_code JOIN department AS T3 ON T2.dept_code   =   T3.dept_code GROUP BY T3.school_code
number of classes offered by each school refers to COUNT(*) GROUPED BY school_code; classes are linked to schools through courses and departments;