I guess it is different depending on your view point but I guess
a grounding in Mathematics is important, I will say Language as everyone needs to understand their native language well (hence lets not say English here)
Science teaches critical thinking skills
Debating
Research skills
Problem solving
Personal finance, budgeting, taxation
even things like cooking is important,
How democracy and other government systems work, how are they similar how do they differ
We do have religious educaiton in the UK, so learning about different religions
A lot of the time education is not seen as relevant, I read somewhere that unless you are studying Physics, then most of what is taught in maths is never used in your life.
How about criminal justice,
How to engage with MPs and or representatives
Having a class representative is good, vote on who that is, vote on school issues, get involved with some decision making within the school through voting, and yes you may vote in favour of something and not get that, well that is how voting works.
Accepting defeat, picking your self up and carrying on.
Activism is also important, but it is also important to be informed so you can make solid arguments to your cause.
I don't have all the answers here, but there are core subjects that make the rest a lot easier, if you learn to read / comprehend information you can learn more by reading and communicating with others. so how to ask questions is another one perhaps.