Worth reading this RSA report:

New data collected from local authorities shows spike in pupils being admitted to schools for excluded pupils in the first term of Year 11, ahead of the January school census that determines if their results will count towards the school’s total.

Read more here: thersa.org/discover/publications-and-articles/rsa-blogs/2019/03/exclusions-exams#

… and in the Times report of the study (paywall): thetimes.co.uk/edition/news/weaker-pupils-removed-before-exams-82cjfhnzr