Yeah, its possible.
Maths especially. If you do all the past papers and questions you can find then its relatively easy to get an A in.
(I should say, i didn't do biology so I'm just speculating but I'm guessing its not to far removed from chemistry)
Chemistry and Biology aren't quite as easy to get better grades in imo but I think its still possible. Again, my advice would be to do all the questions and then more. You need to know the standard responses off by heart and know when to use them which is easy if you become familiar enough with the past papers.
The problem would be in the application questions which can throw you off a bit. Again though, most of these are just another question in disguise so if you do enough past papers and stuff you'll learn to recognise it.
Saying just do past papers makes it sound easy in theory but there is a bit more to it than that. My advice for keeping motivated and doing work is getting some likeminded friends and studying together. That way, its less boring and you'll make sure each other actually does work in addition to helping each other if you get stuck (I personally find explaining and breaking something down helps my understanding of it as well). A lot of A level science (chemistry at least) is just memorising the right answers and fitting them around the context of the question (I'd even include analysis in this). It reaches a point where doing the questions just becomes automatic.
Oh, and get your teacher to mark at least some of them so you don't fall into bad habits. You might think something is right but its not precise enough or whatever in actuality. (Ik i did this because on some exam papers you lost a mark for putting 2 electrons instead of a pair of electrons which is just pedantry imo but its just the way it is i guess)
This worked for me & some friends at least. Idk if it'll work for all types of learner.