OPEN - task 12: Fee Conversion - Clear Tables for Given School 
https://github.com/maine207/aspen/issues/issues/issue/12
Refactored clearTables() method for single schools so that it deletes
table data using SQL statements. The SQL statement deletes the table
data for records that are related to students who currently belong to
the given school. Students with past records at the school but who no
longer belong to the school will have their records skipped during the
delete.