English, Maths, Science, IT, = assessed + compulsory
P.E. + A new philosophy/sociology/politics course = non-assessed + compulsory
That would be my start, the first would make up part of your GCSEs, the second would be a requirement, but would not be graded for the majority of students - unless they chose to take the extended version. so double PE/Double philosophy etc.
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Language is a tricky one. I think it should be compulsory, but at the moment its stupid making it 100% compulsory for GCSE when most students do not arrive at GCSE level with enough ability to ever progress with the language. I own a language education business, and to be honest, youve lost 90% of students already by the time they are 14 in the UK. Over here in China, and in many other countries the key period for language learning is primary school. The middle school (secondary) students that I used to work with here in china, were far far better then the Alevel students I knew in england at their respective languages over here.. You can only make language menaingful and compulsory at GCSE if it becomes a much much bigger focus at primary school. Here it is the 3rd most important subject, they study: Chinese 1st, Maths 2nd, English 3rd, Science 4th.. until they get older, then maths and science swap to the top.. but they understand that its far more bennifical to learn a language a younger age, and a science at an older age.
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As a side note: Maths and English should also be compulsory at Alevel. They are in many parts of the world, and we should not be an exception. Alevels for me should be: Maths + English + 2/3 choices.