Some schools require their students to score at least 75% in their Sec 2 end of year exam, before they are allowed to take additional maths in Sec 3.
For students who are not offered Amath in school and still want to take Amath in O level, they can get the following textbook and assessment book, and self study on their own or find a private tutor to teach them.
Talk to the school and request that the school help you register Amath in O level. The school will probably require you to pass sec 3 end of year examination before they allow you to take Amath at O level.
I have a student who was not offered Amath in school. She wanted to take Amath at O level so she could have more options in future. Her school only allowed her to take the subject on condition that she passed her Sec 3 End of Year A Math exams. The student was not given any opportunity by the school to attend A Math classes. I tutored her and within a short span of 3.5 months, the student had obtained 64 marks (high B4), better than her cohort average of 60.
Though the school now allow her to take Amath at O level, she still cannot take Amath lesson in school, because it clashed with her Principle of Accounts. So I continue to teach her Amath and finished the entire O level syllabus in another 3 months. Eventually she scored A1 in her school sec 4 mid year exams and B3 in O level.
So don’t give up if you really wish to do Amath 🙂