HSPS 🔥 !! The things that you've mentioned may be a bit hard to achieve, but I have no doubt you'll do well
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At Year 10, my advice for now would be to do some further reading around subjects - books, articles, anything that piques your interest! Get involved in essay writing competitions and the like (remember that while winning would be great, the most valuable thing at this stage is gaining experience and getting to write about things that interest you). I've always had an interest in sociology myself - I would recommend a book called Bowling Alone: The Collapse and Revival of American Community by Robert Putnam. I found it to be a very interesting read, so I hope you would enjoy it too
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I know a few people who received offers from Cambridge for HSPS and their main advice would be further reading, getting good at critically analysing texts and strong essay writing skills (under timed conditions can help with some aspects of this, as it would require you to have a structured and concise argument + flow). Also lots of passion
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Something that I would also strongly recommend that I did would be to apply to university outreach programmes, if you qualify. At Year 10 this would be more limited (too ahead of the rest lmao), but I'm sure there should be some available on a smaller scale, like online sessions. MOOCs and virtual experience places are great too! Try places like Springpod and Speakers For Schools - these are the first steps into getting something close to internships, which you can apply for when you're a tad older.
When you're in Year 11, there should be more university programmes available, and in Year 12 there will be a whole lot. In Year 12, there will be residentials available, where you stay at the university for up to a week on a programme for free and immerse yourself in the subject (Cambridge and Oxford are great for these, though do bear in mind that acceptance depends on GCSE grades, widening participation criteria and mini personal statements/ questions). There's also a great UCL summer programme for sociology that they ran for Year 12s last year, where you got to do stuff at the Centre of Longitudinal Studies and visit Parliament (+ free mug and goodies
). But those will have to wait haha, sorry for making you feel like you have to wait - you have a lot in store for you for these kinds of things! And even if you don't get to attend, there's still plenty of things to get involved in, so don't be too concerned.
I also forgot to give the A level run down for each of the subjects you mentioned (this is all anecdotal stuff from friends who do the subject). Psychology is very interesting but can be quite content heavy at times (their notes are STACKED) but the friends I've seen who are passionate about it are really thriving, so hopefully there won't be any issues there. Politics is much the same (you may find that some classmates are especially controversial though lmao). Chemistry is another beast altogether at A level, I must warn you. It's definitely an enjoyable subject if you're interested, but is notoriously challenging for students (I hope this won't deter you, but just a warning beforehand so the transition won't be too surprising)
That's all I can think of right now, but as usual, any questions fire away!! I want to help you as much as I can and give you the guidance I wish I had back then haha