CAGs from teachers with a lot of guidance from exam boards as to what should/shouldn’t be taken into account. Details of the algorithm to be used and how it would be applied (i.e. fairly) made available to all schools in advance to stop people trying to game the system.
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An algorithm based on individual prior attainment (e.g. GCSEs, possibly mock results etc) rather than at cohort level, with details published and buy-in from stakeholders (teachers, parents, unis, student groups), will help students maintain their futures.
The current A Levels and GCSE results will only hinder and disadvantage students' futures.