I’ve just taken advanced higher maths in S5 and achieved an A1 grade. I was studying with a class of S6s.
This was only possible as I self taught higher maths in S4 (sat in a Nat 5 class and could occasionally ask for help from teacher). I,however, do not now have Nat 5 maths although I guess this doesn’t matter.
I attend a state school and I was the first pupil in the school to have done this. As long as the teachers could see I was coping, they were happy.