How is school cafeteria work considered statutory work

You would not be a statutory worker as a school cafeteria worker. 

Statutory workers are independent contractors that by certain laws (or statutes) are required to be treated as employees for payroll purposes.  Generally they include life insurance agents, traveling sales people, and a few other specific industries. If you are a statutory worker, the "statutory employee" box will be checked on your W-2.  

As an employee in a middle school, you are required to work in person at the school and to perform the work on their schedule and according to their rules.  This makes you a traditional employee.  

 Did your W-2 have that box checked?