Seriously...its all up to your school.
During my time,a lot of students got straight As for PMR ,so,they didn stream the students...instead they let the students decide..if those want to be in Pure Sci,they'll get it.But of course,the teacher will look at their results whether they are qualified or not.
So,in the end,there were 5 best classes,those top students from Form 3 were each put in those 5 classes.So,to be the best of the best ,we have to compete among these 5 classes

But to be in Science stream...students should at least get A for Maths and/or Science.Shouldn get C or below for any subjects.