Over £50 million a year is spent by UK schools advertising job vacancies and some £500 million on supply teachers. This is due, largely, to the extortionate fees charged by current suppliers. In 2015, at the ASCL conference, Justine Greening promised this issue would be addressed. It hasn't been.
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Cost of Advertising:
https://www.theguardian.com/education/2016/jul/26/secondary-schools-spent-56m-teacher-job-ads-labour
Cost of Supply Teachers: https://www.telegraph.co.uk/education/2017/02/21/school-spending-supply-teacher-agencies-jumps-fifth-inthree/
Cost of Supply Agencies:
https://www.telegraph.co.uk/education/educationnews/12040309/Schools-spending-thousands-on-agencies-to-recruit-teachers.html
Government Recognition: p24 'Teacher Recruitment and Retention' Briefing Paper 19/0118