Exam invigilator employed by School, not by Exams Board

Trying to establish if I need to submit S/A tax return. My only income is Civil Service Pension (taxed at source), some Bank Interest (less than £1,000 p.a.), and wages while temporarily employed directly by school to invigilate external examinations (taxed at source). The On-Line guide to whether I need to submit a tax return makes a blanket statement about "examiner, exam moderator or invigilator" affecting how I am taxed, but I believe this is directly as a consequence of NIM74100, which distinctly relates to "examiners, moderators, invigilators etc engaged directly by the examining board under a ‘contract of service’ " If someone is solely engaged as an short-term employee by a school, having paid tax and NI through that payroll system, why is there a requirement for an S/A return.

Hi,
If you meet the requirement for the income to be taxed as PAYE income and have no other criteria for a return then one is not required.
Thank you.