That depends on what kind of science you want to do
For astronomy, you should do physics and maths, although I also did chemistry and biology as the skills you learn in each subject come in very useful. Being able to use a computer is also very important, as it is a tool you will end up using a lot.
I actually stopped studying maths as a subject after GCSE, which is quite unusual for a physicist (I still get funny looks even now when I tell people this!)… But my University course taught me the bits of maths I needed to know in order to get my degree, so it is possible to do it without doing it for A-level.
Maths is really important if you want to go and study Physics at university, it’s basically the language of Physics. In fact I would even recommend doing Maths and Further Maths at A-Level (like I did) along with Physics, as you’ll find transitioning into the uni courses a lot easier. Other sciences have different requirements, so if you have an idea of what sort of course you might want to do have a look at some of the requirements in their prospectus or online.
The school subjects you need to become a scientist are basically set by what you need to get into university. Then at university you learn more things that will be useful to get into a PhD programme. An finally after your PhD you can go out and look for a “proper” scientist job! (though at the PhD stage you start being a scientist really).
Since science is something where we always need to compare things like experiment results properly (so quantitatively with numbers), then maths and statistics are very important.
For astronomy you need to do physics, but for something like genetics biology is more useful, so the subjects depend on what kind of science you’re interested in.
Being able to tell your story is equally important, since the most important job of a scientist is to communicate their work to other scientists and to the public. That means that doing english and other subjects that require writing essays and other forms of communication (like giving talks) are also good.
So those are some things to think about for what subjects are important to become a scientist. However, one you are one, the real skill is in putting all those things together to figure out how the Universe works!
Having said all that, I only started my physics degree at university when I was about 24 after doing a technology degree and working for a few years, so it’s not something you have to do straight out of school. The most important thing for being a scientist is to be interested in science.
It depends on what type of scientist you aspire to be. I’ve went from wanting to become a computer scientist, to a mathematician and then to theoretical physicist. Maths is key to a lot of the sciences, especially physics. I found maths hard and boring at school and it was only till I got to college when it started to click and it became fun and interesting. I went from hating it to doing a degree in it and then went onto doing a a PhD in theoretical physics and I got a C in physics at A-level. The physics of the universe is like a poetry written in the pure and beautiful language of mathematics, and so in order to understand the universe you need to be able to do maths and use it as a tool.
If you want to do become a scientist in other areas like biology, chemistry, computing, geology, archaeology etc, then maths is less needed but it is still crucial in some way. The ability to solve problems is crucial for any scientist.
It depends on what you want to do, as others have said. But you can’t go wrong with physics and maths. If you’re good at those two things, you can find paths to other things. I was a Physics major in college, but fell in love with geology and picked up the geology I needed in graduate school… but leveraged my physics/math expertise to do that.