I think teentimestwo is right.
O level days were before my time (2003 when I took gcses) however there was nowhere near the stress there seems to be now, from what I recall. I think the expectations for revision where a lot lower too - we would be studying syllabus up until around Easter, then off for study leave. Maybe a couple of revision lessons in each suject before breaking up. It seems very full on revision from Feb half term at the latest, sometimes after Christmas, now.
BUT when I was sitting them, a lot of vocational schemes would have their own maths or English assessments for candidates who didn't have a grade C or above. Minimum gcse requirements to get on were 5 Es.
Even for A levels, at both our school sixth form and college you would be fine with Cs in your A level subject at GCSE (In terms of getting in - whether you'd cope with the actual A level is perhaps a bit different).
I'm sure we all know people who left school with a handful or no GCSEs and have gone on to have a successful career, starting at the bottom and working their way up. I just don't know if this generation's teens will get the chance to do that when just about every job now wants C in maths and English.
So it's a lot of pressure for a sixteen year old, who isn't, at sixteen, particularly academic.