What are the different school codes?
select distinct school_code from department
For each one, how many professors does it have?
select t1.school_code, count(*) from department as t1 join professor as t2 on t1.dept_code = t2.dept_code group by t1.school_code
Which one has the most?
select t1.school_code from department as t1 join professor as t2 on t1.dept_code = t2.dept_code group by t1.school_code order by count(*) limit 1