Do grades derive purely from our intellect at GCSE? And A-Level?

I'm interested to know what people think on what influences our academic attainment. I'll avoid reasons why - that deserves its own thread - but not entirely. To reflect the relative importance of each, put a percentage mark on each. (The numbers will likely be arbitrary, but the relative proportions are what matter most).
A precis of the meaning behind the figures would be good too. Below are my own indicators, but use any you like (mine are examples):

GCSE
Student's agency, intellect, reasoning skills, memory etc 40%
Skill/competency of teacher 15%
School administration/bureaucracy 5%
Physical school environment 15%
Home environment or family/socioeconomic background 35%

A-Level
Student's agency, intellect, reasoning skills, memory etc 50%
Skill/competency of teacher 10%
School administration/bureaucracy 10%
Physical school environment 10%
Home environment or family/socioeconomic background 20%

With small but appreciable differences between secondary school and A-Level, basically I reckon that conventional wisdom is wrong. Our grades aren't mostly down to our own efforts and intelligence.