Mine is quite a niche one, but...manipulating my O Level subject options so that I didn't take any of the science subjects. I can't see any teenager doing GCSEs getting away with that now!
This was at a grammar school back in the late 1970s. All three sciences were taught and examined as separate subjects. At the end of Lower Fifth/Year 10 when we made our O Level choices we could drop up to two of them. Those of us who weren't especially good at maths were advised to drop physics and chemistry but keep going with biology as it was the "least maths-y".
...However, for people who were particularly gifted at languages, already in the top ability group for French, and who'd chosen the year before that to continue with Latin to O Level (rather than drop Latin and pick up either German or home economics for three years, to O Level), there was the option at the end of Upper Fourth/Year 9 to drop one science subject and pick up a two-year compressed course for O Level German. I'd already done that, dropping physics.
So at the end of Lower Fifth, I "innocently" announced my intention to drop two science subjects and got shot of chemistry and biology as well. None of the staff noticed until midway through Upper Fifth/Year 11 when they filed the exam paperwork and by then it was far too late for them to insist I should try and catch up with a further five months' work in either subject.
Despite dire prognostications that I was sabotaging my entire future by committing myself to A Life Without Science Qualifications - especially as it wasn't at all certain I'd pass O Level maths - I carried on into sixth form, got A Levels, passed various professional exams, later did a degree through the OU, and had a long and satisfying career. 