In the infamous English education system, a core subject is one which is required at GCSE level. Every student in the country must take exams in English (often separate language and literature examinations), Mathematics and Science (either Biology, Chemistry and Physics separately, or amalgamated in a single exam), plus a number of optional subjects chosen by the student. A modern foreign language such as French or German is also required, as is a form of Design and Technology, but these are not classed as core subjects.
A large proportion of the time spent in primary education is dedicated to these core subjects, as evidenced by the new literacy and numeracy hours introduced by the Labour government. The basic theory is that every child should get their three r's - reading, (w)riting and (a)rithmetic - with a dash of how things work thrown in. Much that is English is based on time-honoured values, but this is simply because they are the ones that work best.