Depends on how much work you did for GCSEs tbh. I got 4 A's at GCSE including chemistry, and I point blank refused to work throughout all of Yr 11 for Chemistry. I didn't revise, I didn't hand in homework, I purposely scheduled guitar lessons over chemistry. And bamn, it was my best GCSE.
A-levels, came to my mocks and I got BDUU. They were a big shock for me as I'd always managed to half-ass it and still get reasonable grades.
With GCSEs, without revision you can generally get grades that more or less reflect your ability in each subject (hence why mine were good, but not amazing). With A-levels, you start the year with a U grade in each subject and you essentially have to fight for each grade up. How easy that is depends on how well you used your lesson time, if you're revising effectively, and if you're at all good in exams.
As for being in the lessons, for most subjects I found the content wasn't necessarily harder, there was just more of it, which made coming to exam time harder.