Depends. If your teacher is setting you a bit of reading for you to do so lesson time could be used for consolidation (ngl I always do this minutes before the lesson but I plan on doing better for Year 12 and 13) or anything like that, I think it's fair and I'd do that.
If, at A-Level, they're still setting trash like my KS4 science teacher's favourite - posters, leaflets and research barely relevant to a rushed curriculum even when you squint, I don't blame you if you let it go through one ear and out the other.
But you really gotta stop slacking during A-Level for the most part. We can't swim through A-Levels like we did at GCSE, I've already begun to notice that.