Why do British high schools have such a huge impact on the performance of students in university?

I am studying in a UK top university as an international student who went to a public school. I feel that I have to work much harder than my classmates for lower marks, as I am always tired, I have less time for extracurricular activities. My marks are decent, but my classmates who went to private schools get higher marks with the same amount of effort.

Why do British high schools have such a huge impact on the performance of students in university?
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