[mercury_craze]

The story I've heard a few times is that students who are expected to perform poorly in their GCSE's or A-Levels get expelled for minor behavioural infractions prior to their final exams so their poor grades don't risk bringing down the average.

[Traster]

It’s more complicated than that. Stage 1 - you can’t do subjects they don’t think you’ll do well in. Stage 2 you won’t be “offered” a place studying for GCSEs if you didn’t do well in the previous year. Stage 3, If you get bad GCSEs you won’t return for A levels. Stage 4, you get an offer from a prestigious university but get caught smoking pot (or some equally expellable act) you’ll be invited to study from home for the rest of the year so they don’t damage their statistics and don’t need to report your behaviour.